diff --git a/docs/envvars.rst b/docs/envvars.rst
index 7085bcf7..e9316c64 100644
--- a/docs/envvars.rst
+++ b/docs/envvars.rst
@@ -77,6 +77,20 @@ Building
 
 Allows to set :ref:`projectconf` option :ref:`projectconf_build_flags`.
 
+Examples:
+
+.. code-block:: bash
+
+    # Unix:
+    export PLATFORMIO_BUILD_FLAGS=-DFOO
+    export PLATFORMIO_BUILD_FLAGS="-DFOO -DBAR=1 -DFLOAT_VALUE=1.23457e+07"
+    export PLATFORMIO_BUILD_FLAGS="'-DWIFI_PASS=\"My password\"' '-DWIFI_SSID=\"My ssid name\"'"
+
+    # Windows:
+    SET PLATFORMIO_BUILD_FLAGS=-DFOO
+    SET PLATFORMIO_BUILD_FLAGS=-DFOO -DBAR=1 -DFLOAT_VALUE=1.23457e+07
+    SET PLATFORMIO_BUILD_FLAGS='-DWIFI_PASS="My password"' '-DWIFI_SSID="My ssid name"'
+
 .. envvar:: PLATFORMIO_SRC_BUILD_FLAGS
 
 Allows to set :ref:`projectconf` option :ref:`projectconf_src_build_flags`.
diff --git a/docs/projectconf.rst b/docs/projectconf.rst
index 09a49eb5..2f1c368a 100644
--- a/docs/projectconf.rst
+++ b/docs/projectconf.rst
@@ -301,7 +301,10 @@ Example:
 .. code-block::   ini
 
     [env:specific_defines]
-    build_flags = -Dfoo -Dbar=1
+    build_flags = -DFOO -DBAR=1 -DFLOAT_VALUE=1.23457e+07
+
+    [env:string_defines]
+    build_flags = '-DHELLO="World!"' '-DWIFI_PASS="My password"'
 
     [env:specific_inclibs]
     build_flags = -I/opt/include -L/opt/lib -lfoo
diff --git a/platformio/builder/tools/platformio.py b/platformio/builder/tools/platformio.py
index 1be95fb8..32418b50 100644
--- a/platformio/builder/tools/platformio.py
+++ b/platformio/builder/tools/platformio.py
@@ -96,17 +96,19 @@ def BuildProgram(env):
 
 
 def ProcessFlags(env, flags):
-    parsed_flags = env.ParseFlags(flags)
-    for flag in parsed_flags.pop("CPPDEFINES"):
-        if isinstance(flag, list):
-            env.Append(
-                CPPDEFINES=[
-                    '-D%s=\\"%s\\"' % (flag[0], flag[1])
-                ]
-            )
-        else:
-            env.Append(CPPDEFINES=flag)
-    env.Append(**parsed_flags)
+    for f in flags:
+        if not f:
+            continue
+        parsed_flags = env.ParseFlags(str(f))
+        print parsed_flags
+        for flag in parsed_flags.pop("CPPDEFINES"):
+            if not isinstance(flag, list):
+                env.Append(CPPDEFINES=flag)
+                continue
+            if '\"' in flag[1]:
+                flag[1] = flag[1].replace('\"', '\\\"')
+            env.Append(CPPDEFINES=[flag])
+        env.Append(**parsed_flags)
 
     # fix relative CPPPATH
     for i, p in enumerate(env.get("CPPPATH", [])):