The purpose of this study was to compare the data/model fit for two competing theories of the conceptual roles that Negative Affectivity (NA) and Positive Affectivity (PA) play in the stressor-strain relationship. In the ‘trait model’, NA is understood to be a confounder that inflates the perceived work-related stressor-outcome relationship, while PA is unrelated to either stressors or strain. Alternatively, the ‘situational model’ assumes that NA and PA are directly affected by stressors and are thought to mediate the stressor-relationship. The sample consisted of 731 Swedish engine room officers. Role stress was used as a stressor indicator, perceived stress was the outcome measure, and the PANAS was used to assess levels of affectivity. The path analysis gave strong support for the work situational model (RMSEA = 0.034) while no support was found for the trait model. No moderating effects from affectivity were found.