Background: Schwartz (1987) suggested that three discrete sub-processes may be involved in the production of the thematic structure of sentences. These are: (1) The retrieval of the semantic representations of the main lexical items; (2) The specification of the predicate argument structure (PAS); and (3) The assignment of the lexical items to thematic roles within the PAS. There has been no comprehensive investigation of the three aspects of processing in the performance of individual people with aphasia. Aims: This study aimed to investigate the presence of the three sub-processes by determining whether they can be differentially impaired in aphasia. Methods & Procedures: Four people with aphasia (GW, JM, KD, and TJ) who had apparent difficulties in producing thematic structure were included in the study. They presented with similar surface symptoms in connected speech: a high percentage of single phrases, limited production of complex three-argument structures, and the omission of obligatory arguments. Their performance on various tests of single word and sentence processing was compared to that of normal control subjects and the pattern of errors analysed. Outcomes & Results: The clients presented with different patterns of impaired and retained performance and different patterns of error. This suggested that different underlying impairments were responsible for their poor production of thematic structure. All four clients presented with some verb retrieval difficulties, although only GW and TJ's deficits were of a semantic nature. TJ also had difficulty understanding and retrieving nouns, but when given the words showed awareness of the PAS and could assign thematic roles appropriately. JM presented with a specific difficulty specifying the PAS, and KD had a specific difficulty with thematic role assignment. GW had difficulties both with the specification of PAS and thematic role assignment. Conclusions: The results of the study suggest that difficulties in producing the thematic structure of sentences may be a consequence of different underlying impairments. The different impairments provide some support for the sub-processes suggested by Schwartz. The same surface symptoms in connected speech can be a consequence of different underlying impairments and thus if therapy is be targeted at the impaired process, treatment needs to be preceded by detailed assessment.