Suicidal ideation is a major risk factor for suicidal behavior and has recently been shown to be fluctuating in studies applying ecological momentary assessments (EMAs). The aims of this study are to introduce a reliable and valid item set for assessing suicidal ideation and relevant proximal risk factors that can be used in future EMA studies within suicidology. Additionally, we provide data on the ability of the items to capture moment-to-moment variability and discuss feasibility aspects of EMA studies on suicidal ideation in a clinical sample. Psychiatric inpatients diagnosed with a depressive disorder (N = 74) rated a set of 28 items measuring suicidal ideation, thwarted belongingness, perceived burdensomeness, depression, anxiety, hopelessness, and positive affect 10 times per day on 6 consecutive days. In addition, these constructs were assessed by self-report questionnaires before and after the EMA period. Intraclass correlations and mean squared successive differences were calculated as indicators of item variability. Reliability was calculated at the prompt (within-person) and the person (between-person) level, applying an approach based on multilevel factor analysis. Convergent validity was assessed by correlating the EMA scores with a self-report questionnaire measuring the same constructs. All items demonstrated moment-to-moment-variability and substantial within-person variance. Moreover, all items and scales, except those assessing anxiety, showed satisfying reliability at the prompt and the person level, and correlations indicated convergent validity of the EMA item set. Compliance with the EMAs was excellent (89.7%). Researchers are encouraged to apply this useful tool in future EMA studies in the field of suicidology. (PsycINFO Database Record (c) 2018 APA, all rights reserved).