Time-dependent quantum reaction dynamics calculations using eight degrees of freedom are reported in a study of the isotopic reaction, D-2 + C2H, on a new modified potential energy surface. It shows that vibrational excitations of D-2 enhance the reactivity, whereas the bending excitations of C2H hinder the reactivity. The comparison of the three isotopic reactions also shows the isotopic effect in the initial-state-selected reaction probability, integral cross section and rate constants. The rate constant comparison shows that the D-2 + C2H reaction has the smallest reactivity among the three isotopic reactions D-2/HD/H-2 + C2H reaction, then HD + C2H, and H-2 + C2H has the largest. (C) 2010 Elsevier B.V. All rights reserved.