Mechanically and thermomechanically stressed interface cracks in adhesively bonded bimaterials (PMMA-aluminum) with a large elastic and thermal property mismatch are experimentally studied. The elasto-optic effects are mapped as (sigmax+sigmay) contours in the PMAA halves and interfacial fracture parameters are estimated. Crack initiation under mechanical and thermomechanical loading conditions are shown to be controlled by different micromechanical processes. The results suggest that the micromechanical unlocking of microcavities and microprotrusions along the interface is primarily responsible for failure initiation under thermomechanical loading conditions. This is unlike the mechanical loading situations wherein fracture toughness is derived primarily from the breakage of interlocking microentanglements. The measured values of the fracture parameter DeltaIm(Kaiepsilon)T due to a temperature rise is a constant and much higher than its real counterpart (DeltapsiT (a) approxequal 76-82 degrees). The Delta(Kaiepsilon)T|cr thus obtained are much lower than the mechanical counterparts.