The potential alkali-silica reactivity of volcanic rocks of rhyolitic composition, used as concrete aggregates, was assessed focusing mainly on their petrography. Eleven samples from the Argentine Patagonia (Provinces of Río Negro and Chubut) were examined using a petrographic microscope. Thin sections were used to evaluate sample texture, degree of alteration, the presence of volcanic glass in the matrix, and metastable minerals that may contribute silica to the reaction (ASR). Results from the petrographic study were compared with those from the mortar bar test (ASTM C-1260) and the dissolved silica determined by the chemical methods (ASTM C-289). It was concluded that rhyolitic rocks may develop deleterious reactions when they contain unweathered and/or altered glass in their matrices, and alteration minerals resulting from devitrification such as argillaceous minerals and cryptocrystalline silica. Rhyolitic rocks in which the matrix has completely recrystallized were found to be non-reactive.