Three proton transfer (PT) reactions (a) HC(H)CH+ + C2H2 --> C2H2 + HC(H)CH+, (b) HC(H)CH+ + C2H4 --> C2H2 + H2C(H)CH2+, and (c) H2C(H)CH2+ + C2H4 --> C2H4 + H2C(H)CH2+ are theoretically studied. High-level calculations show that the electron correlation plays a crucial role in such processes. The direct pi -attack PT mechanism in the cross arrangement is revealed, and symmetrical and asymmetrical PT pathways are suggested between the proton-bound C2H2 and C2H2 (or C2H4) moieties and between two proton-bound C2H4 moieties, respectively. This study may provide the first theoretical results on the pi -proton-donor to pi -acceptor PT systems.