ABSTRACTWe investigate the design of harm‐based and act‐based sanctions for environmental offences. Empirical evidence concerning both criminal and administrative sanctions for Belgium clearly shows that the determination of harm‐based and act‐based sanctions for environmental offences is less straightforward than theory predicts. In criminal cases, harm‐based sanctions are influenced by offence‐related factors and specifically by the environmental harm caused, while the level of act‐based sanctions is found to be independent of offence characteristics. Further, offender characteristics have a similar impact on both types of sanction, with the exception of the treatment of corporate offenders. In administrative cases, both types of sanction positively depend on the seriousness of (potential) harm caused. We find that sanctions increase for repeat offenders and decrease for offenders who took actions to minimize (potential) damages. Moreover, the analysis provides evidence of multiple objectives pursued by enforcing authorities. While our results confirm the general insights from previous studies, they are innovative in the distinction made between harm‐based and act‐based sanctions. Copyright © 2014 John Wiley & Sons, Ltd and ERP Environment