Moiré interferometry is employed to study toughening in partially stabilized zirconia (PSZ). Energy to fracture as a function of crack growth curves (R-curves) is derived from mode I compliance calculations and from near tip fitting of the moiré fringes. The effect of the tetragonal to monoclinic phase transformation in the zirconia is found by comparing the bulk compliance R-curves to the locally derived moiré R-curve. Localized strain field plots are produced from the moiré data for the PSZ zirconia. The observed transformation zone height compares favorably with that predicted by Okada et al. [Acta metall. mater. 40, 1421 (1992)] in a companion paper, as does the qualitative nature of the R-curve with predictions by Stump and Budiansky [Acta metall. 37, 3297 (1989)].