The chemical weathering of rock proceeds by water-rock interaction. During weathering geochemically mobile elements, alkali and alkali-earth elements are easily leached from rocks, leaving the residual elements to combine with components from the atmosphere to form new minerals. No single weathering index given in the literature fits the model of the process involved in chemical weathering outlined above. This study deals with the derivation of three new chemical weathering indices by taking both chemical processes such as leaching and new mineral formation into account. The new indices are chemical leaching index (CLI), chemical weathering product index (CWPI) and total chemical weathering index (TCWI). These indices are applied to selected weathering profiles in the Kurtun granodiorite, NE Turkey, in order to estimate the effects of chemical weathering on the physico-mechanical properties of rock materials. The relative variation of the mechanical properties and TCWl display a statistically significant correlation. Besides, it is shown that TCWl and weatherability index, originally defined by Hodder (1984) can be used together to evaluate the effect of chemical weathering on the mechanical behaviour of rocks.