Integrating signalling theory and the portfolio diversity literature, we theorize that diversity in a firm's patent and alliance portfolios sends contrasting flow signals impacting its market value in a nuanced way. Diversity in an alliance portfolio mediates the patent portfolio diversity – market value relationship by suppressing the negative effect of patent portfolio diversity creating an overall positive effect. We test our mediation model on a longitudinal set of 225 US biopharmaceutical firms that were awarded 17,078 patents and participated in 37,744 alliances between 1990 and 2006. Our theory and findings contribute three novel insights. First, we demonstrate the value of a temporal lens in explaining why diversity in a firm's patent and alliance portfolios send flow signals that establish expectations among market observers and have performance implications. Second, establishing that patent and alliance portfolio diversity are temporally sequenced provides compelling evidence for the value of studying multiple types of portfolios, their temporal relationships and effects on firm outcomes. Third, since diversity in a firm's portfolios can send contrasting flow signals conditioned on the cognitive demands and proximity involved in interpreting the signals, firms that do not maintain a ‘signalling fit’ with market observers increase the probability of unintentional negative signalling effects.