We have recently reported high efficiencies in a monolithic III–V triple-junction solar cell design that is grown inverted with a metamorphic 1.0 eV bottom In.27Ga.73As junction. The biaxial stress and strain grown into this highly lattice-mismatched junction can be controlled by varying the design of a step-graded GaxIn1−xP buffer layer, in which most, but not all, of the 1.9% misfit strain is relieved. A multi-beam optical stress sensor (MOSS) is a convenient tool for in situ measurement of stress during metal–organic vapor phase epitaxy (MOVPE) for the optimization of solar cell performance. The analysis of stress from curvature data is complicated by significant temperature effects due to relatively small thermal gradients in our atmospheric-pressure MOVPE reactor. These temperature effects are discussed and approximations made to allow practical analysis of the data. The results show excellent performance of inverted In.27Ga.73As solar cells grown with slight compressive stress, but degradation under tensile stress. The best devices had a Voc of 0.54 V and a dislocation density in the low 106 cm−2. The in situ stress data is also compared with ex situ strain data derived from X-ray diffraction measurements.