We have investigated the properties of wafer-fused Si-doped isotype heterojunctions between GaAs and InP. Current/voltage measurements were conducted to study the influence of the doping concentrations on each side of the interface on the electrical conductivity. An almost ohmic behavior with a very low series resistance was obtained for the highest examined doping level on the GaAs side, whereas the doping concentration on the InP side was found to be of little significance. Fusion at different temperatures showed that the conductivity degrades significantly below 500°C, although mechanically stable junctions were obtained also at temperatures as low as 305°C. Secondary ion mass spectroscopy measurements showed no redistribution of Si, but indicated the presence of small amounts of C and Fe impurities at the interface.