From Raman scattering on a-plane wurtzite ZnO crystals we obtained a decreasing splitting between longitudinal and transversal optical phonons with A1 and E1 symmetry as a function of hydrostatic pressure up to 5.5 GPa. Consequently, the transverse effective charge e T exhibits a strong reduction with increasing pressure, yielding 2.17–14.6x10E−3 P/GPa and 2.04–13.7x10E−3 P/GPa in units of the elementary charge for the A1 and E1 phonons, respectively. We find a clear systematic in the linear pressure coefficient of eT* with bond polarity for the series of wide-band gap semiconductors SiC, AlN, GaN, and ZnO.