Small-angle neutron scattering has been used to study the microstructure of natural porous basalt rocks. The effect of temperature on the rock microstructure has been investigated on ‘as received’ and heat-treated basalts. The magnitudes of α, the power-law scattering exponent were between 3 and 4 for the majority of the rocks, indicating a surface fractal structure between the basalt matrix and the pore space. Heat-treated basalts show higher α values, and therefore a smoother pore surface. Internal surface areas were determined for all basalts depending on the thermal history.