This paper introduces a simple, first principles-based model describing the liquid holdup in the catalyst-containing pockets of Katapak-SP, a modular catalytic structured packing developed to allow a certain degree of flexibility with respect to the variation in reaction-to-separation requirements in a single unit. The basic requirement for the catalyst-containing pockets in this respect is to be fully filled with flowing liquid which implies that the operating holdup is bound between the static holdup of the catalyst bed as the lower end, and that corresponding to the upper limit, the so-called catalytic load point. The latter is the liquid load corresponding to the bed saturation point, indicating that excessive liquid will be retained, i.e., will remain in the separation part of the packing element and mix with the liquid leaving the catalyst-filled pockets at the bottom of the element. Detailed knowledge of the liquid holdup as well as the pattern of the trickling flow is essential because it governs the performance of the reaction part and consequently the hybrid unit as a whole. Both glass and resin (an industrial catalyst) particles were used in conjunction with water and a binary water-methanol mixture as working fluids. The model predictions for static holdup and the catalytic load agree well with the experiments.