A sponsor portfolio exists where multiple brands sponsor a single activity or property, such as a sporting event, team, league, or a charity simultaneously. While sponsor portfolios are common in practice, little is known about how the brand perceptions of several concurrent sponsors spill over to influence each individual sponsor’s brand. This paper summarizes two experiments that investigate sponsor portfolios to determine how spillover effects influence consumers’ perceptions of a particular sponsor’s brand within the portfolio. In Study 1, empirical evidence substantiates a brand spillover effect between multiple sponsors of a single sport property. In Study 2, the influences of image congruence and portfolio size on this spillover effect are empirically assessed. Results demonstrate an interaction effect whereby brands incongruent to the sponsored property enjoy a more favorable brand perception when included in either a small portfolio inclusive of another incongruent co-sponsor, or a larger portfolio of otherwise congruent sponsors