A series of LaAl11O18- and Al2O3-supported LaCrO3 and Cr2O3 combustion catalysts was prepared. Different active phase-support combinations were prepared and applied to cordierite monoliths. The washcoat materials were aged in flowing humid air at temperatures between 1100 degrees C and 1400 degrees C, after which they were characterized by BET, XRD, TPR, and EDS. The monolith catalysts were evaluated in methane combustion. The presence of an active phase retarded sintering of the Al2O3 support, whereas the active phase slightly decreased the thermal stability of LaAl11O18. X-ray measurements revealed extensive interaction between support and active phase in the washcoat materials. A substituted perovskite, LaCr1-xAlxO3, is proposed to be formed in nearly all samples containing both lanthanum and chromium. The accessibility of chromium decreased rapidly after aging. The activities of the Al2O3-supported catalysts were higher than of those supported on LaAl11O18, which was related to the higher surface area of the former.