The effective activation energies for the self-propagating high-temperature synthesis of Ti5Si3 were determined for samples with porosities ranging from 30% to 62%. A low-activation-energy regime was found for highly porous samples, while relatively high effective activation energies of 200-250 kJ/mol were measured far samples with porosity less than 50%. The same effect was also observed in two other systems, involving syntheses of TiC and MoSi2. The limiting role of interparticle heat transfer in propagation of the combustion front for highly porous samples is suggested as a possible explanation for the observed phenomenon.