The optical band gap in 40 nmGa1−xInxN/GaNsingle heterostructures is investigated in the composition range0<x<0.2by photoreflection spectroscopy (PR) at room temperature and compared with photoluminescence (PL) data. Clear PR oscillations at the GaInN band gap are observed as originating in the large piezoelectric field. Effective band gap bowing parametersbare derived for pseudomorphically stressed GaInN on GaN:b=2.6 eV(PR) andb=3.2 eV(PL in localized states). Using experimental deformation potentials of GaN,b=3.8 eVis extrapolated for the optical band gap in relaxed GaInN material. Previously reported smaller values are discussed. ©1998 American Institute of Physics.