Acid Rock Drainage (ARD) prediction is complicated by the number and complexity of influencing parameters. The objective of this study was to determine the effect of particle size and mineral liberation on the production of ARD. This information was used to classify waste rocks from an Australian mine site for their potential use as cover material. Samples were crushed and sieved into different particle size fractions, and were subjected to varying static and kinetic tests including paste-pH, Acid-Neutralization-Capacity tests, kinetic Net Acid Generation tests and Humidity Cell tests. The results showed a strong influence of particle size, which can be attributed to the widely different mineral liberation characteristics of acid generating and neutralizing minerals. Whereas standard Acid Base Accounting (ABA) resulted in an uncertain or Non Acid Forming classification of open-pit waste rocks, the application of modified ABA test procedures using different particle size fractions resulted in an uncertain or Potentially Acid Forming classification of the very same material. The study demonstrated that the standard ABA procedure may lead to erroneous classification with potentially costly environmental consequences by overlooking particle size specific mineral liberation effects. Testing different particle size classes proved to be a practicable method to investigate the ARD characteristics of waste rocks and to gain a clearer insight into the consequences of mineral liberation.