Strained layer (111)B-grown In0.15Ga0.85As/GaAs p-i-n quantum well structures have been investigated by thermally-detected optical absorption and modulation spectroscopy in order to determine the piezoelectric constant in the (In,Ga)As layers, The impact of both indium segregation and temperature is shown to be great on this determination. We show that those two phenomena may account for the discrepancy between the values reported in the literature and that expected from a linear interpolation between the piezoelectric constants of the two binaries InAs and GaAs.