Small-angle neutron scattering (SANS) and scanning electron microscopy (SEM) have been used to study the microstructure of several volcanic rocks (basalt, rhyolite, phonolitic pumice, phonolite, rhyolitic obsidian). Magnitudes of the power-law scattering exponent were 3 < α < 4 or α ≈ 4 for the majority of the rocks. The influence of the thermal history on the rock microstructure has been investigated by SANS experiments on thermally treated samples. Specific surface areas were determined for pumice (3 m2/g) and a basalt (2.5 m2/g and 0.2 m2/g) depending on the thermal history. The small-angle scattering in obsidian is shown to be caused by Fe-rich partially crystalline aggregates. The obsidian has been further characterized by ultrasonic resonance and thermal expansion measurements.