Both normal and aphasic subjects produce picture naming errors that are semantically related to the target. This study investigates the effects of different word properties on the production of semantic errors by aphasic subjects. A correlation is found between the production of semantic errors and deficits in the comprehension of high-imageability words, with those patients who make semantic errors in comprehension also producing them in naming. This suggests an absence of deficits resulting in semantic errors only in word production. For some patients, semantic errors are more likely to occur with lower-imageability targets, despite the restricted range of imageability values for stimuli in this picture naming task. However, not all of those patients who produce semantic errors show an effect of imageability on their occurrence, suggesting a dissociation between central conceptual/semantic deficits and post-semantic deficits. The majority of patients fail to show frequency effects in the production of semantic errors, contrary to the predictions of most models of spoken word production. However, for some of those patients who have a central semantic deficit, there is an interaction between frequency and imageability, with a frequency effect for low-imageability items alone. It is argued that this is the more appropriate analysis rather than the study of main effects. The problems that the diverse patterns of presence and absence of frequency effects cause for current theoretical models of spoken word production are discussed.