A new diffusion Monte Carlo study is performed on the isomers Of C4H3 and C4H5 emulating the methodology of a previous study (Int. J. Chem. Kinet. 2001, 33, 808). Using the same trial wave function form of the previous study, substantially different isomerization energies were found owing to the use of larger walker populations in the present work. The energy differences between the E and i isomers Of C4H3 were found to be 10.5 +/- 0.5 kcal/mol and for C4H5, 9.7 +/- 0.6 kcal/mol. These results are in reasonable accord with recent MRCI and CCSD(T) findings.