Product involvement is an important factor in consumer evaluation of co-branding. However, academic discussions over this factor have largely been neglected. This paper bridges this gap and investigates how product involvement moderates the strength of the reciprocal effect on each brand partner. We validate two theory-driven propositions by using a theoretical modeling approach. Proposition 1 explains that the negative reciprocal effect on partnering brands is stronger in the scenario of higher-involved categories than in lower-involved categories. Proposition 2 argues that the positive reciprocal effect on partnering brands can be more significant in the case of lower-involved categories than in higher-involved categories. To the best of our knowledge, we are the first to explore the influence of product involvement on reciprocal effects in the scenario of a moderately-incongruent co-branding alliance.