This thesis deals with the determination of the effect of controlled relative humidity (RH) on the mechanical properties of coil coatings. The coil coatings studied were based on polyester/melamine technology. The study employed mechanical and dynamic mechanical analysis, using modified environmental chambers, to measure the effects of controlled RH on a range of mechanical parameters. Of particular interest was the effect of controlled RH on the minimum forming temperature of the coil coated metallic panels. This is the temperature below which the coating would fracture. A description has been given of the modifications made to both a mechanical tester and a Dynamic Mechanical Analyser (DMA), which allowed controlled RH experiments to be performed under isothermal or thermal scanning conditions. Comparison of this study with published work showed that some experimental data in the literature could be erroneous due to lack of, or inappropriate, RH conditioning prior to testing. The results showed a good correlation for both free film and coated metal samples between the glass transition temperature (Tg) and the brittle-tough transition (Tbt). A reduction of both the Tg and the Tbt of up to 15°C were found as the controlled RH was increased. A controlled RH and environmentally instrumented slow-draw cupping device was constructed to measure the behaviour of coil coatings on aluminium (Al) and hot dip galvanised steel (HDG-steel) substrates. The importance of indentation depth during the cupping test and the difference between the substrates was emphasised. Fracture mechanics was employed to study differences in performance.