We present measurements on terahertz generation by optical rectification, in which both the thickness of the generation crystal and the wavelength of the generating optical pulse are varied. The difference between the group velocity at optical frequencies and the phase velocity at terahertz frequencies affects the time trace and spectrum of the generated terahertz pulse. For the thickest crystal, we find that the phase mismatch gives rise to the generation of two pulses, separated by 4.5 ps. The physical origin of the observed features is clarified with a simple model that includes phase matching and absorption.