The group of Alvand granitic rocks is distributed widely around the city of Hamadan, western Iran, and a number of important civil engineering structures have been founded on and in this rock group. Thus, the weathering characteristics of Alvand granites are of considerable practical importance. The granites are generally medium to coarse-grained and porphyritic, light-grey to buff in colour. To assess the influence of weathering on the geomechanical characteristics of these rocks, a sampling and testing program was carried out. Field work involved assessment of the rock mass weathering grades, execution of simple field tests and collection of representative samples from different weathering grades. This paper presents the results of laboratory investigations, including weathering characteristics and physical, chemical and mechanical properties. Interrelationships are determined for all test results. From the field and laboratory studies, robust linear relationships are found between the weathering degree of the tested samples and their geomechanical properties.