Loading of acetaldehyde from the gas phase into transition metal substituted aluminophosphate sieves of AFI structure resulted in mainly physisorbed CH3CH=O in the case of FeAlPO4-5. By contrast, FT-IR spectroscopy revealed extensive chemical reaction upon loading into metal-free AlPO4-5 and CoAlPO4-5 materials at temperatures as low as -100 degrees C. The low temperature product is identified as acetaldol, which dehydrates to crotonaldehyde upon warm-up to room temperature. The reactive sites are proposed to be Lewis acid sites in the aluminophosphate framework. Using FT-IR monitoring of acetaldehyde in the pores of aluminophosphate sieves as a sensitive method for the detection of Lewis acid sites, we find that their abundance varies substantially with metal substitution. Interestingly, isomorphous Fe substitution of the aluminophosphate framework seems to suppress the formation of such defect sites.