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This paper describes some experimental work in which the visual effects of incorrect decoding of the chrominance signals in colour television receivers were assessed by panels of observers. There are difficulties in changing circuit parameters in an actual receiver during a viewing session to simulate decoding errors. Apparatus was therefore constructed to produce exactly similar effects to those given by an incorrect receiver. Preliminary tests were conducted using this apparatus to show the effects of several different kinds of decoding error. For these tests the N.T.S.C. X/Z decoding system was used. Following these tests, the effects of amplitude errors in the output colour difference signals used to drive the display tube were investigated in greater detail. The later tests are therefore relevant to all of the three currently proposed systems. The results of these experiments were compared with other workers' theoretical calculations, thereby allowing some assessment to be made of the visual importance of the calculated colour errors.