Uniaxial compression tests and SEM analyses were conducted on Tertiary and Jurassic outcrop mudrocks after exposure to different water- and oil-based fluid phases. The tests were conducted as part of an investigation into the role of rock-fluid interactions on drilling and wellbore problems. It was found that increasing intensity of rock-fluid interactions led to a decrease in brittleness of the mudrocks as well as reductions in their strengths and stiffnesses. Interestingly, on exposure to the most reactive fluids (water and 4% NaCl brine), the more competent Jurassic mudrock experienced a greater percentage reduction in strength than the Tertiary material, even though its clay content comprised a lower proportion of swelling minerals. It was demonstrated that behaviour similar to that seen for the mudrocks in their natural states could also be achieved using selected water based systems as well as oil-based muds.