diff --git a/Foundation/include/Poco/FPEnvironment_C99.h b/Foundation/include/Poco/FPEnvironment_C99.h
index 74f819c05..b94ecb5a3 100644
--- a/Foundation/include/Poco/FPEnvironment_C99.h
+++ b/Foundation/include/Poco/FPEnvironment_C99.h
@@ -31,22 +31,22 @@ class FPEnvironmentImpl
 protected:
 	enum RoundingModeImpl
 	{
-#ifdef FE_DOWNWARD
+#if defined(FE_DOWNWARD)
 		FP_ROUND_DOWNWARD_IMPL   = FE_DOWNWARD,
 #else
 		FP_ROUND_DOWNWARD_IMPL   = 0,
 #endif
-#ifdef FE_UPWARD
+#if defined(FE_UPWARD)
 		FP_ROUND_UPWARD_IMPL     = FE_UPWARD,
 #else
 		FP_ROUND_UPWARD_IMPL     = 0,
 #endif
-#ifdef FE_TONEAREST
+#if defined(FE_TONEAREST)
 		FP_ROUND_TONEAREST_IMPL  = FE_TONEAREST,
 #else
 		FP_ROUND_TONEAREST_IMPL  = 0,
 #endif
-#ifdef FE_TOWARDZERO
+#if defined(FE_TOWARDZERO)
 		FP_ROUND_TOWARDZERO_IMPL = FE_TOWARDZERO
 #else
 		FP_ROUND_TOWARDZERO_IMPL = 0
@@ -54,27 +54,27 @@ protected:
 	};
 	enum FlagImpl
 	{
-#ifdef FE_DIVBYZERO
+#if defined(FE_DIVBYZERO)
 		FP_DIVIDE_BY_ZERO_IMPL = FE_DIVBYZERO,
 #else
 		FP_DIVIDE_BY_ZERO_IMPL = 0,
 #endif
-#ifdef FE_INEXACT
+#if defined(FE_INEXACT)
 		FP_INEXACT_IMPL        = FE_INEXACT,
 #else
 		FP_INEXACT_IMPL        = 0,
 #endif
-#ifdef FE_OVERFLOW
+#if defined(FE_OVERFLOW)
 		FP_OVERFLOW_IMPL       = FE_OVERFLOW,
 #else
 		FP_OVERFLOW_IMPL       = 0,
 #endif
-#ifdef FE_UNDERFLOW
+#if defined(FE_UNDERFLOW)
 		FP_UNDERFLOW_IMPL      = FE_UNDERFLOW,
 #else
 		FP_UNDERFLOW_IMPL      = 0,
 #endif
-#ifdef FE_INVALID
+#if defined(FE_INVALID)
 		FP_INVALID_IMPL        = FE_INVALID
 #else
 		FP_INVALID_IMPL        = 0
@@ -109,37 +109,61 @@ private:
 //
 inline bool FPEnvironmentImpl::isInfiniteImpl(float value)
 {
+#if POCO_OS == POCO_OS_AIX
+	return ::isinf(value) != 0;
+#else
 	return std::isinf(value) != 0;
+#endif
 }
 
 
 inline bool FPEnvironmentImpl::isInfiniteImpl(double value)
 {
+#if POCO_OS == POCO_OS_AIX
+	return ::isinf(value) != 0;
+#else
 	return std::isinf(value) != 0;
+#endif
 }
 
 
 inline bool FPEnvironmentImpl::isInfiniteImpl(long double value)
 {
+#if POCO_OS == POCO_OS_AIX
+	return ::isinf((double) value) != 0;
+#else
 	return std::isinf((double) value) != 0;
+#endif
 }
 
 
 inline bool FPEnvironmentImpl::isNaNImpl(float value)
 {
+#if POCO_OS == POCO_OS_AIX
+	return ::isnan(value) != 0;
+#else
 	return std::isnan(value) != 0;
+#endif
 }
 
 
 inline bool FPEnvironmentImpl::isNaNImpl(double value)
 {
+#if POCO_OS == POCO_OS_AIX
+	return ::isnan(value) != 0;
+#else
 	return std::isnan(value) != 0;
+#endif
 }
 
 
 inline bool FPEnvironmentImpl::isNaNImpl(long double value)
 {
+#if POCO_OS == POCO_OS_AIX
+	return ::isnan((double) value) != 0;
+#else
 	return std::isnan((double) value) != 0;
+#endif
 }