An experimental investigation was carried out into (a) the incidence and kind of overt verbalisation in the perfor­mance of "non-verbal" tasks, and, (b) the occurrence of "displacement behaviour" during the per- formance of these The importance of language in the solution of non-verbal tasks has been demonstrated on normal children. The present study is concerned with two different age groups of subnormal children. A series of tasks was presented to the subjects, their performance being recorded by audio-visual equipment. Subsequent analysis of the data showed that there were:- (1) statistically significant differences between the kinds of verbalisation - relevant and irrelevant. (2) no significant differences between the two age groups in their verbal behaviour, though certain trends were apparent in terms of task difficulty and kinds of verbalisation. (3) significant differences between the two age groups with respect to displacement behaviour during tasks of high difficulty. (4) significant differences in some of the measures of displace­ment behaviour between tasks of low and high difficulty. General explanations of the results are offered and discussed. explanations concern the type of analysis employed, the nature of the sample, the difficulty levels of the tasks, and the roles played by the types of verbalisation in problem-solving behaviour.