Real-time observations of film stress are presented from three growths of strained InGaAs/GaAs layers. The initial metastable growth regime is analyzed to extract the alloy compositions of x = 0.136, 0.155. and 0.180. The strain values at the end of the growths (thicknesses 0.791, 1.08, and 0.115 mum, respectively) are then used to deduce the amounts of relaxation gamma = 0.808, 0.857, and 0.261, respectively. These data obtained from the in situ optical stress sensor are compared with conventional measurements using asymmetric x-ray diffraction. The agreement is good, within 4%-6% for composition, indicating that the in situ sensor is suitable for quantitative study of strain relaxation during film growth.