Firmly held beliefs that have a delusional quality are commonly experienced in those with schizophrenia spectrum disorders (SSD) and have been reported in those with dissociative identity disorder (DID). However, no study to date has compared delusional belief content and characteristics between these diagnostic groups. This study examined delusional content, and the degree of conviction, preoccupation and distress associated with them in 50 participants with DID and 50 with an SSD exploring also dissociation and childhood trauma as predictors of delusional beliefs. Multivariate analysis of variance and linear regressions were conducted to explore differences between beliefs and characteristics and to examine its association with dissociation and childhood trauma. The SSD sample presented more selfreferential delusional beliefs and characteristics compared to the DID group. Yet, the DID group had more mistrust delusional beliefs and characteristics in comparison to SSD participants. Mistrust beliefs were predicted by depersonalization/derealization in the DID sample, but did not predict any delusional belief in the SSD sample. The content of fixed beliefs differs between DID and SSD samples and in this study depersonalization/derealization experiences were related to mistrust beliefs but not to other delusional forms, and only in the DID sample.