In this paper we review our recent results from optical and magneto-optical studies which have allowed identification of the dominant radiative and some non-radiative (NR) recombination processes in Ga(In)NAs and GaNP alloys and related quantum structures. The dominant mechanism for photoluminescence (PL) in Ga(In)NAs is determined as recombination of excitons trapped by potential fluctuations of the band edge, induced by compositional disorder and strain non-uniformity of the alloy. The alloy fluctuations are shown to be largely related to non-optimized growth conditions and can be suppressed by increasing growth temperature or using post-growth thermal annealing. On the other hand, the PL emissions in GaNP alloys are concluded, to arise from deep states likely related to N clusters whereas the band-to-band recombination is predominantly NR. With increasing N composition the defect States are shown to become resonant with the conduction band of the alloy and thus optically inactive, leading to an apparent red shift of the PL maximum position. Experimental evidence for a band crossover from an indirect to a direct band gap in GaNP at very low N compositions (similar to0.5%) is provided from the transient studies of the PL transitions. The first available experimental information on the chemical identity and formation mechanism of NR defects in Ga(In)NAs is also presented. (C) 2002 Elsevier Science Ltd. All rights reserved.