This paper discusses how incidences of unit taxes and ad valorem taxes are related to various market characteristics under imperfect competition. Incidence is higher the more competitive markets are. More specifically, it is higher the wider the firms' product ranges, the lower the degree of market concentration and the lower the degree of product differentiation, and higher in Bertrand markets compared to Cournot markets. Furthermore, the incidence is higher for unit taxes than for ad valorem taxes. The relative difference between unit and ad valorem tax incidence is larger the lower marginal costs and taxes are.