It is important to recognize the effect of weathering in the performance of rock in engineering works. The aim of this study was to determine the weathering characteristics of acidic volcanic rocks from northwest Turkey and their effects on some engineering properties of the rocks. A series of simple and multiple regression and factor analyses suggested that porosity, indirect tensile strength, block punch strength and P wave velocity were the most suitable parameters for a quantitative estimation of weathering grades for engineering purposes. A weathering classification system for the acidic volcanic rock materials was developed based on rating these four engineering parameters. In addition, a weathering classification system based on visual descriptions and Schmidt hammer rebound values is also suggested for the rock masses studied.