Promotion of CaO by Na+ or Li+ leads to an active and selective catalyst for the oxidative coupling of methane into higher hydrocarbons. The structural changes of NaOH- and LiOH-impregnated Ca(OH)(2) during calcination were studied by applying differential thermal analysis, IR and X-ray diffraction techniques. The results indicate that both NaOH and LiOH interact with Ca(OH)(2) during drying and calcination. The alkali hydroxides tend to dehydrate along with Ca(OH)(2) into oxides. In situ infrared spectroscopic studies suggest that dehydration of the mixed hydroxides occurs around 450 degrees C. The measured lattice parameter values indicate that the inclusion of Na+ or Li+ ions into the CaO matrix remains very limited. It may be assumed that most of the alkali oxide remains in well dispersed state over the CaO matrix when calcining the samples at 600 degrees C. On increasing the alkali content above 10% no further increase in hydrocarbon selectivity is achieved.