The purpose of this study was to identify characteristics of investigations of sexual abuse concerning vulnerable adults residing in facility settings that were associated with case substantiation. Data on 410 reports of sexual abuse were collected prospectively from Adult Protective Services (APS) and state licensure agency staff in New Hampshire, Oregon, Tennessee, Texas, and Wisconsin over a six-month period. Specifically, we examined differences between reports that were substantiated and those that were not by comparing characteristics of alleged victims, alleged perpetrators, and aspects of investigation using logistic regression. We found that a relatively low proportion of cases (18%) were substantiated overall. Compared to cases that were not substantiated, cases that were substantiated were more likely to feature nursing home residents, older victims, female victims, and allegations of physical contact between the alleged perpetrator and victim. Despite the high proportion of alleged perpetrators who were facility staff (51%) compared to resident perpetrators (25%), cases with resident-to-resident allegations of abuse were much more likely to be substantiated, accounting for 63% of substantiated cases. In light of these findings, we believe it is important that investigators are trained to handle sexual abuse cases appropriately and that they are able to investigate the case thoroughly, promptly, and with as much information as possible. It is also critical that investigators make substantiation decisions using the appropriate standard for confirmation (e.g., preponderance of the evidence, beyond a reasonable doubt, clear and convincing evidence) as state law dictates.