A fully instrumented and bench-mounted diamond drilling machine, purpose designed and built, has been used for investigating the effect of aqueous solutions of various chemical compounds when used as drilling fluids. The rocks investigated were andesite, basalt and granite, the chemical compounds were alkylamines, with various hydrocarbon chain lengths, aluminium chloride and dioxane. Rotation speed seems to be one of the major factors affecting the emergence of mechanochemical effects as the results obtained with aluminium chloride clearly indicate. Penetration rate is either unaf- fected or enhanced by the presence of certain compounds, such as aluminium chloride. On the other hand, penetration rate diminishes when the drilling fluid contains dioxane and exhibits an apparently unpredictable behaviour with alkylamine solutions, although it appears that an increase in hydrocarbon chain length usually equates to an increase in penetration 1. Full Professor, Mining and Mineral Dressing Department University of Cagliari - Italy 2. Associate Professor, Mining and Mineral Ikea- sing Department University of Cagliari - Italy rate. Rock texture seems to have a pronounced effect on test repeatability. With hexadecy- lamine acetate solutions in concentrations ranging from 10-5 to 5 .10-4 mol dm 3 and 41.88 rads-1 rotation speed, 46 % increments in penetration rate of andesite were observed, with respect to water, whereas penetration rate decreased in basalt by as much as 60%o when a dioxane solution was used as drilling fluid. An interpretation of the observed effects is proposed in the light of current knowledge of solid state physics and physical Chemistry.