Existing research on co-branding focused only on co-branding effects on new products and brand attitude at the beginning point of cooperation, without further study on changes occurring after cooperation. On the basis of a review and compilation of the relevant literature, this paper explored how, in a co-branding alliance, the negative information about one co-brand impacts the other co-brand, involving brand popularity as a moderator. This paper also considered the concept of involvement to investigate the effects of co-branded negative information under different involvement levels. The results showed that (1) Negative information about a co-brand decreased consumer attitudes toward that co-brand as well as toward the co-produced product, and then consequently eroded consumer attitudes toward the other co-brand in the alliance; (2) The negative information about a co-brand with high popularity was more influential on the other co-brand in the alliance than was the negative information about the co-brand with low popularity; (3) Given that co-brand A was more popular than co-brand B in an alliance, the negative information about A had a stronger impact on B when their popularity difference was large than when the popularity difference was small. However, when co-brand A was less popular than co-brand B, the negative information about A had a stronger impact on B when their popularity difference was small than when it was large; (4) The effect of co-brand A?s negative information on consumer attitudes toward co-brand B was smaller for high-involved consumers than for low-involved consumers. Key Words: Co-branding, attitudes, negative information, brand awareness differentiation, involvement.