Formaldehyde gas loaded into a framework Fe aluminophosphate sieve (FeAIPO(4)-5) at 250 K was found to react with adsorbed H2O, CH3OH, H2O2, or lattice OH groups to yield the corresponding addition products, namely, CH2(OH)(2), CH3OCH2OH, HO2CH2OH, or POCH2OH, respectively. Reactions were monitored in situ by static FT-IR spectroscopy, and assignments are based on experiments with CD2=O and CD3OD. Most efficient was the reaction with H2O2, as indicated by the fact that Ho2CH,OH was formed at the exclusion of CH2(OH)(2) and POCH2OH when adsorbing formaldehyde onto a sieve loaded with H2O2 and H2O. Methoxymethanol, methanediol, and POCH2OH were stable at 250 K but dissociated above 0degreesC under release of formaldehyde. Hydromethyl hydroperoxide disproportionates to formic acid and water. Under 355 nm irradiation in a FeAlPO4 sieve, HO2CH2OH was found to undergo efficient photofragmentation.