The triplet metastable states of acetylene produced by intersystem crossing from the (A) over tilde((1)A(u)) state have been detected by a sensitized phosphorescence (SP) method. The phosphorescence was observed from C2H2 in the energy region below the barrier to dissociation in the I state suggested by Hashimoto and Suzuki [J. Chem. Phys. 1996, 104, 6070]. The lifetimes of the triplet states coupled with the (VK1)-K-3 and (VK1)-K-4 levels in the (A) over tilde((1)A(u)) state were estimated to be 80 and 100 mu s. The rotational structures of the laser-induced fluorescence and SP spectra were similar, except that the enhancement of many weak absorption lines makes the latter more congested. The feature of the SP spectrum exemplifies the complicated singlet-triplet mixing at the (VK1)-K-3 level. The SP signal was observed with different phosphors, where benzil was found to be the most sensitive. The SP spectrum of C2D2 was observed only by using benzil. In addition to SP, the collision of metastable triplet acetylene with a phosphor surface yielded fluorescence. This is most likely to be fluorescence from the (A) over tilde((1)A(u)) state of acetylene. The lifetime of the triplet states and the threshold energy to dissociation in C2H2 and C2D2 suggest that tunneling occurs on the a state and that the energy barrier to dissociation is higher than the previous estimation.