The fluorenylidene-substituted phosphaalkene MesP=Fl (13) and arsaalkene MesAs=Fl (14) (Mes = 2,4,6-t-Bu(3)C(6)H(2), Fl = 2,7-di-tert-butylfluorenylidene) have been prepared by treatment of Fl(H)Li with MesECl(2) (E = P, As) followed by dehydrochlorination with DABCO (1,4-diazabicyclo[2.2.2]octane). In the synthesis of 13 it was possible to isolate the intermediate, Mes(Cl)P-(H)Fl (11). Attempts to prepare the stibaalkene, MesSb=Fl (15), resulted in an unstable product. Compounds 11, 13, and 14 were characterized by X-ray crystallography: 11, triclinic, P&onemacr;, a = 11.548(2) Å, b = 12.189(3) Å, c = 14.223(3) Å, alpha = 114.13(1) degrees, beta = 92.79(1) degrees, gamma = 107.14(1) degrees, V = 1712.8(2) Å(3), Z = 2, wR2 = 0.1344, R1 = 0.0560; 13, monoclinic, P2(1)/n, a = 24.597(7) Å, b = 11.185(2) Å, c = 26.914(7) Å, beta = 112.16(2) degrees, V = 6857(3) Å(3), Z = 8, wR2()()= 0.1477, R1 = 0.0794; 14.Et(2)O, monoclinic, P2(1)/n, a = 15.577(4) Å, b = 15.274(4) Å, c = 17.255(5) Å, beta = 108.17(2) degrees, V= 3901(2) Å(3), Z = 4, wR2()()= 0.1275, R1 = 0.0578. The P=C (1.686(5) Å) and As=C (1.807(3) Å) bond distances in 13 and 14 are consistent with double-bonding descriptions; moreover, there is a zero angle of twist between the E-C(Mes) moieties and the plane of the Fl() ligand in both molecules.