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Law pressure MOVPE has been used to grow a series of GaN epilayers, deposited onto (0001) sapphire substrates. All the growth parameters have been kept constant, except for the V/III molar ratio, which has been changed from sample to sample by changing the ammonia flow rate. The growth rate has been measured vs the V/III molar ratio at both low temperature (550 degrees C, growth of the buffer layer) and normal process temperature. A strong dependence of the growth rate vs the V/III molar ratio is found at both temperatures. Systematic photoluminescence and reflectivity experiments have been performed at 2 K on all samples. The photoluminescence is dominated by the donor bound exciton and donor acceptor pair are visible on samples grown under the lowest V/III molar ratio, indicating higher impurity incorporation. An optimum V/III molar ratio of 5000 is proposed, which corresponds to the highest growth rate observed and to the lowest full width at half maximum for the photoluminescence line. (C) 1997 Elsevier Science Ltd.