diff --git a/models/cost.cc b/models/cost.cc
index 1936900..cc23a61 100644
--- a/models/cost.cc
+++ b/models/cost.cc
@@ -28,8 +28,8 @@ modes 1 = URBAN, 2 = SUBURBAN, 3 = OPEN
 http://morse.colorado.edu/~tlen5510/text/classwebch3.html
 */
 	if (f < 150 || f > 2000) {
-		printf
-		    ("Error: COST231 Hata model frequency range 150-2000MHz\n");
+		fprintf
+		    (stderr,"Error: COST231 Hata model frequency range 150-2000MHz\n");
 		exit(EXIT_FAILURE);
 	}
 
diff --git a/models/ecc33.cc b/models/ecc33.cc
index 642234f..c08cb0c 100644
--- a/models/ecc33.cc
+++ b/models/ecc33.cc
@@ -11,7 +11,7 @@ double ECC33pathLoss(float f, float TxH, float RxH, float d, int mode)
 	}
 
 	if (f < 700 || f > 3500) {
-		printf("Error: ECC33 model frequency range 700-3500MHz\n");
+		fprintf(stderr,"Error: ECC33 model frequency range 700-3500MHz\n");
 		exit(EXIT_FAILURE);
 	}
 	// MHz to GHz
diff --git a/models/ericsson.cc b/models/ericsson.cc
index 0082958..66c8d20 100644
--- a/models/ericsson.cc
+++ b/models/ericsson.cc
@@ -12,8 +12,8 @@ double EricssonpathLoss(float f, float TxH, float RxH, float d, int mode)
 	double a0 = 36.2, a1 = 30.2, a2 = -12, a3 = 0.1;
 
 	if (f < 150 || f > 3500) {
-		printf
-		    ("Error: Ericsson9999 model frequency range 150-3500MHz\n");
+		fprintf
+		    (stderr,"Error: Ericsson9999 model frequency range 150-3500MHz\n");
 		exit(EXIT_FAILURE);
 	}
 
diff --git a/models/los.cc b/models/los.cc
index d4655f8..778e017 100644
--- a/models/los.cc
+++ b/models/los.cc
@@ -150,7 +150,7 @@ namespace {
 
 		int rc = pthread_create(&threads[thread_count], NULL, rangePropagation, arg);
 		if (rc)
-			printf("ERROR; return code from pthread_create() is %d\n", rc);
+			fprintf(stderr,"ERROR; return code from pthread_create() is %d\n", rc);
 		else
 			++thread_count;
 	}
@@ -161,7 +161,7 @@ namespace {
 		for(unsigned int i=0; i<thread_count; i++) {
 			int rc = pthread_join(threads[i], &status);
 			if (rc)
-				printf("ERROR; return code from pthread_join() is %d\n", rc);
+				fprintf(stderr,"ERROR; return code from pthread_join() is %d\n", rc);
 		}
 		thread_count = 0;
 	}
diff --git a/models/sui.cc b/models/sui.cc
index 113962c..03549c9 100644
--- a/models/sui.cc
+++ b/models/sui.cc
@@ -16,7 +16,7 @@ double SUIpathLoss(float f, float TxH, float RxH, float d, int mode)
 	 */
 	d = d * 1000;		// km to m
 	if (f < 1900 || f > 11000) {
-		printf("Error: SUI model frequency range 1.9-11GHz\n");
+		fprintf(stderr,"Error: SUI model frequency range 1.9-11GHz\n");
 		exit(EXIT_FAILURE);
 	}
 	// Terrain mode A is default