diff --git a/bower.json b/bower.json
index a75c17d..9626072 100644
--- a/bower.json
+++ b/bower.json
@@ -12,12 +12,14 @@
     "output"
   ],
   "dependencies": {
+    "purescript-arrays": "master",
     "purescript-effect": "master",
     "purescript-foreign-object": "master",
     "purescript-maybe": "master",
     "purescript-node-streams": "master",
     "purescript-posix-types": "master",
     "purescript-unsafe-coerce": "master",
-    "purescript-prelude": "master"
+    "purescript-prelude": "master",
+    "purescript-st": "master"
   }
 }
diff --git a/src/Node/Process.js b/src/Node/Process.js
index 27b3de7..df0531e 100644
--- a/src/Node/Process.js
+++ b/src/Node/Process.js
@@ -46,20 +46,6 @@ exports.chdir = function (dir) {
   };
 };
 
-exports.setEnv = function (var_) {
-  return function (val) {
-    return function () {
-      process.env[var_] = val;
-    };
-  };
-};
-
-exports.unsetEnv = function (var_) {
-  return function () {
-    delete process.env[var_];
-  };
-};
-
 exports.exit = function (code) {
   return function () {
     process.exit(code);
diff --git a/src/Node/Process.purs b/src/Node/Process.purs
index e20e31c..00cbc45 100644
--- a/src/Node/Process.purs
+++ b/src/Node/Process.purs
@@ -6,11 +6,14 @@ module Node.Process
   , onUncaughtException
   , onUnhandledRejection
   , argv
+  , argv'
   , execArgv
+  , execArgv'
   , execPath
   , chdir
   , cwd
   , getEnv
+  , env
   , lookupEnv
   , setEnv
   , unsetEnv
@@ -27,6 +30,10 @@ module Node.Process
 
 import Prelude
 
+import Control.Monad.ST.Global (Global)
+import Control.Monad.ST.Global as STGlobal
+import Data.Array.ST (STArray)
+import Data.Array.ST as STArray
 import Data.Maybe (Maybe)
 import Data.Posix (Pid)
 import Data.Posix.Signal (Signal)
@@ -34,6 +41,8 @@ import Data.Posix.Signal as Signal
 import Effect (Effect)
 import Effect.Exception (Error)
 import Foreign.Object as FO
+import Foreign.Object.ST (STObject)
+import Foreign.Object.ST as STObject
 import Node.Platform (Platform)
 import Node.Platform as Platform
 import Node.Stream (Readable, Writable)
@@ -86,15 +95,22 @@ onSignal sig = onSignalImpl (Signal.toString sig)
 nextTick :: Effect Unit -> Effect Unit
 nextTick callback = mkEffect \_ -> process.nextTick callback
 
--- | Get an array containing the command line arguments. Be aware
--- | that this can change over the course of the program.
+-- | Get an array containing the command line arguments.
 argv :: Effect (Array String)
-argv = mkEffect \_ -> process.argv
+argv = STGlobal.toEffect (STArray.freeze argv')
 
--- | Node-specific options passed to the `node` executable. Be aware that
--- | this can change over the course of the program.
+-- | Get the mutable array containing the command line arguments.
+argv' :: STArray Global String
+argv' = process.argv
+
+-- | Node-specific options passed to the `node` executable.
 execArgv :: Effect (Array String)
-execArgv = mkEffect \_ -> process.execArgv
+execArgv = STGlobal.toEffect (STArray.freeze execArgv')
+
+-- | Mutable array of node-specific options passed to the
+-- | `node` executable.
+execArgv' :: STArray Global String
+execArgv' = process.execArgv
 
 -- | The absolute pathname of the `node` executable that started the
 -- | process.
@@ -111,18 +127,24 @@ cwd = process.cwd
 
 -- | Get a copy of the current environment.
 getEnv :: Effect (FO.Object String)
-getEnv = mkEffect \_ -> process.env
+getEnv = STGlobal.toEffect (FO.freezeST env)
+
+-- | Mutable reference to the current environment.
+env :: STObject Global String
+env = process.env
 
 -- | Lookup a particular environment variable.
 lookupEnv :: String -> Effect (Maybe String)
-lookupEnv k = FO.lookup k <$> getEnv
+lookupEnv k = STGlobal.toEffect (STObject.peek k env)
 
 -- | Set an environment variable.
-foreign import setEnv :: String -> String -> Effect Unit
+setEnv :: String -> String -> Effect Unit
+setEnv k v = void $ STGlobal.toEffect (STObject.poke k v env)
 
 -- | Delete an environment variable.
 -- | Use case: to hide secret environment variable from child processes.
-foreign import unsetEnv :: String -> Effect Unit
+unsetEnv :: String -> Effect Unit
+unsetEnv k = void $ STGlobal.toEffect (STObject.delete k env)
 
 pid :: Pid
 pid = process.pid