This study explores the impact of non-constant returns to scale and imperfectly competitive product markets on the empirical performance of the q-model of investment. We present an econometric specification linking the unobserved shadow price of capital to financial market data when returns-to-scale are not necessarily constant and product markets are not necessarily competitive. Furthermore, unlike most Q-studies, we work within a discrete-time framework, and give explicit attention to the relations between inventories, sales, and production. Our new specification allows us to recover parameters characterizing returns-to-scale, product market, and adjustment cost elasticities. Estimates are reported for 12 mature industries in the United States drawn from VALUELINE.