In this paper, we consider the role of imperfect competition in explaining the relative price of non-traded to traded goods within the Balassa-Samuelson framework. Under imperfect competition in the two sectors, relative prices depend on both productivity differentials and mark-up differentials. We test this implication using a panel of sectors for the seven major OECD countries. The empirical evidence suggests that relative price movements are well explained by productivity and mark-up differentials. Unlike the original Balassa-Samuelson model, aggregate demand could affect the real exchange rate by changing the mark ups. The empirical results show that aggregate demand fluctuations lead to changes on the mark-ups
