Thermolysis of highly diluted (CH2Se)(3) or (CD2Se)(3) in a flow of argon with subsequent quenching of the products in an matrix at 15 K yields monomeric CH2Se and CD2Se, respectively. Six fundamental vibrations v(1) = 2972.5, v(2) = 1413.3, v(3) = 854.2, v(4) = 916.4, v(5) = 3052.9, and v(6) = 913.2 cm(-1) and two combination bands have been observed for CH2Se as well as three fundamentals of CD2Se. The vibrational wavenumbers are compared with those of CH2O and CH2S. Matrix isolated selenoformaldehyde decomposes slowly by UV photolysis to yield the CSe molecule.