The 20-item Partner Interaction Questionnaire (PIQ-20) is frequently used to assess social support for adults wanting to stop smoking. Given that social support may play a significant role in quitting success, there is a need to understand the structure and psychometric properties of assessment instruments designed to measure the construct of partner support. The current study examined the psychometric properties of the PIQ-20 when used to assess the frequency of partner behaviors. The study participants included 380 adult volunteers (M age = 41 years, SD = 12; 58% male). To assess internal consistency, we used both the traditional coefficient-alpha and the latent variable modeling composite reliability (coefficient-ρ) procedures. We conducted independent factor analytic methods to address issues of dimensionality and scoring of responses to the PIQ-20 items. Also, we used an item response theory modeling procedure to examine the specificity of scores on the items. Reliability estimates for the PIQ-20 subscale scores were adequate (values ≥.70). The bifactor analysis supported deriving a total score for each subscale. Item response theory modeling demonstrated that the discrimination (a-slope) parameter for each subscale item was significantly different from zero. The majority of items were associated strongly with their respective subscales. Twelve items were identified that could be adopted as a potential short form of the PIQ-20. The PIQ-20 or short form provides an opportunity for assessing positive and negative partner support simultaneously. There is empirical support for the dimensional structures and scoring of responses for both versions of the instrument. (PsycINFO Database Record (c) 2019 APA, all rights reserved).