Purpose – The effect of personality traits on impulse purchase is not well understood. The purpose of this paper is to investigate how two such traits, shyness and sociability, impact on impulse buying tendency. Understanding drivers of impulse purchase, a significant source of retail sales, is important to succeed in the challenging environment that store-based retailers face. Design/methodology/approach – A customer intercept approach was taken to collect 194 responses from two locations in a busy city centre shopping street in Dublin, Ireland, and the hypotheses were tested using covariance-based structural equation modelling. Findings – Sociability has a significant positive effect on both affective and cognitive impulse buying tendencies whereas shyness has a positive effect on cognitive impulse buying tendencies only. Age was found to be a significant moderator of these effects. Research limitations/implications – This study uses an in vivo street-intercept approach and hence the results may differ for other data collection approaches. The lack of discriminant validity of the sub-scales for impulse buying tendencies has implications for future research. Practical implications – The findings suggest that retailers should factor customers’ psychological traits into their decision making. Individuals with high levels of sociability and shyness are prone to buying on impulse but in different ways. Younger people are more likely to exhibit these behaviours. Originality/value – This paper meets a need for research on how customers’ traits affect their impulse purchase tendencies, and offers retailers advice on how to attract and serve customers with these traits.