Total cross-sections (TCSs) for electron and positron scattering from C3H4 isomers (allene and propyne) were measured using the linear transmission method in the range of 0.8-600 eV for electrons and 0.7-600 eV for positrons. Elastic cross-sections for electron scattering were also measured by a crossed-beam experiment in the range of 2-100 eV. Sharp resonance-like peaks were observed in the electron TCSs for propyne around 3.6 and 8 eV, and 2.5 and 11 eV for allene. The positron TCSs for these molecules were found to be smaller and much smoother than the electron TCSs. Clear differences in resonance positions and magnitudes between the two isomers can be viewed as an isomer effect. (C) 2002 Elsevier Science B.V. All rights reserved.