Whilst the Hackman and Oldham job characteristics model (JCM) continues to attract research attention, including questions about its factorial structure, very few have questioned its comprehensiveness. The model postulates five job dimensions, but it is questioned whether these are necessary and sufficient for the development of the outcomes which the model predicts. This study investigates the effect of including a sixth dimension, a measure of person-task match called self-expression. Data from two surveys of public sector employees (n = 170 and n = 160) were used to test the hypotheses that self-expression will make a unique contribution to the variance explained in both job satisfaction and job involvement beyond that explained by the five job characteristics of task identity, task significance, autonomy, skill variety, and feedback and that self-expression will be a stronger influence on job involvement than it will be on job satisfaction. The hypotheses were partly supported, with self-expression tending to show stronger relationships with job involvement than with job satisfaction. These results support O'Brien's (1985) argument that the JCM is not comprehensive without the inclusion of a person-task match variable.