Abstract Objective. Previous research on illness perceptions has focused on single illnesses, but most adults over 65 suffer from multiple illnesses (multimorbidity). This study tests three competing operationalisations of multiple illness perceptions in predicting physical functioning and adherence: (1) main effects and interactions model, (2) peak model with effects of the most prominent illness perception, and (3) combined model with averaged illness perceptions over multiple illnesses. Design. Longitudinal study in N=215 individuals (65-86 years) with multimorbidity at two measurement points over six months. Participants filled in two Brief Illness Perception Questionnaires (B-IPQ) on their two most severe illnesses. Main outcome measures. Physical functioning, medication adherence. Results. Factor analyses suggest that the B-IPQ contains three dimensions; consequences, control, and timeline. Multiple regression models fit the data best for (1) and (3). Timeline (β=-.18) and control (β=.21) predict adherence in (1); consequences (β=.16) and control (β=.20) in (3). Physical functioning was significantly predicted by interacting control beliefs in (1; β=.13), by peak consequences in (2; β=-.14), and by consequences (β=-.15) in (3). Conclusions. Individuals with multimorbidity hold both distinct and combined perceptions about their illnesses. To understand individual responses to multimorbidity, perceptions about all illnesses and multimorbidity as entity should be examined.