Whilst assessment tools have been developed to diagnose schizophrenia in people with mild intellectual disabilities (IDs), little attention has been paid to developing reliable and valid dimensional measures of psychotic experiences with this population. This study investigates the reliability and validity of two such measures developed for the general adult psychiatric population, the Positive and Negative Syndrome Scale (PANSS) and the Psychotic Symptom Rating Scales (PSYRATS), with a population of adults with mild IDs. Sixty-two adults with mild IDs were interviewed using the PANSS and PSYRATS, and independently interviewed using the Psychiatric Assessment Schedule -- Adults with Developmental Disability (PAS-ADD) to obtain psychiatric diagnoses to the criteria of the International Classification of Diseases -- Tenth Revision (ICD-10). On the basis of ICD-10 diagnosis, participants were divided into three groups: psychosis (n = 11); other mental health problem (n = 14); no mental health problem (n = 37). PANSS and PSYRATS subscale scores were compared across these three groups and were correlated with PAS-ADD symptom scores across a number of PAS-ADD symptom domains. All PANSS and PSYRATS subscales showed adequate internal reliability, largely good test-retest reliability, and logical inter-correlations between subscales. The PANSS positive symptoms and the PSYRATS auditory hallucinations subscales differentiated between the psychosis group and the other groups; the PANSS general symptoms subscale differentiated between the psychosis and no mental health problem groups; and the PANSS negative symptoms and the PSYRATS delusions subscales did not differentiate between the three groups. The PANSS and PSYRATS are promising measures for use with people with mild IDs and psychotic experiences, although further investigation of items relating to negative symptoms and delusions is warranted.