The theoretical calculations at the MP2/DZ+d level have been carried out for the formation and the reaction of a series of ylides of group 15 elements, H(3)M double bond CH2, M = N, P, As, Sb, and Bi. First, the preparation of ylides by means of the salt method was evaluated : MH(3) + CH,Cl-3 --> H(3)M(+)-CH3 + CT, and H(3)M(+)-CH3 + OH- --> H(3)M double bond CH2 + H2O. The ylide formation was most feasible for P, but it is highly endothermic for N and Bi. These ylides have in general larger ylide rather than ylene character, and the carbanion character increases in the order P < As < Sb < Bi. The N ylide is different from others in that it shows no double bond character at all; the N-C bond in H3N-CH2 is even longer than that in H2N-CH3. Two reaction routes of these ylides with formaldehyde were then investigated : H(3)M double bond CH2 + H2C double bond O --> ethylene + H(3)M double bond O (Wittig type) and H(3)M double bond CH2 + H2C double bond O --> ethylene oxide + MH(3) (Corey type). The Wittig reaction is favored both kinetically and thermodynamically for the P ylide. For the As, Sb, and Bi ylides, on the other hand, the Wittig-type reaction is favored kinetically but the Corey-type reaction is thermodynamically preferred. These differences are attributed to the relative oxygen affinity and the H(3)M double bond CH2 bond strength of these group 15 elements. The N ylide is very reactive in the Corey-type reaction due to the weak H3N double bond CH2 bond, but no intermediate nor transition state was found in the Wittig route. The different reaction pattern of the N ylide was primarily attributable to the larger electronegativity of N compared to P, As, Sb, Bi, and H.