Many previous studies of transport tax reform have explicitly or implicitly assumed that the reform itself does not affect the marginal value of time. In this paper we consider a simple model with multiple trip purposes, commuting and non-commuting transport, to analyse the implications of transport tax reform for the value of time and for marginal external congestion costs. The theoretical analysis shows that transport taxes may both increase or reduce the value of time and it identifies the conditions under which either outcome will occur. The results further suggest that if a tax reform in the transport sector is accompanied by labour tax adjustments to reduce the distortionary cost of the tax system, the marginal value of time will typically increase. The implications of endogenous values of time for models of optimal externality taxes and studies of tax reform in the transport sector are empirically illustrated using a numerical model, calibrated using Belgian data. It is found that the impact of tax changes on the value of time is non-trivial. Moreover, many of the tax reform exercises considered simultaneously reduce traffic levels but raise marginal external congestion costs. The results of this paper suggest that incorrectly assuming exogenous time values may strongly bias optimal congestion taxes and lead to misleading welfare effects of transport tax reform.