Unsaturated polyester resins are extensively used in the automotive industry as matrix for polymer based composite materials, but have poor impact resistance. Toughness can be improved dispersing a softer phase in brittle polymers. Scrap tyres constitute a significant waste problem and are a low-cost source for rubber. The objective of this work is the mechanical characterization of matrix systems containing solid rubber particles obtained from scrap tyres. Several pre-treatments were tested based on contact time and temperature. The flexural and Charpy impact behaviour of the modified resins showed a decrease in performance. Dynamic mechanical tests did not result in clear evidence of phase mixing. Optical microscopy showed the presence of contaminant particles, which were considered the cause for the low performance. Among the modified resins, a positive shift in flexural behaviour occurred when pre-heating was used. It is thought the strategy may prove effective in enhancing resin toughness, if the waste rubber is purified prior to its utilization.