The current study had three aims. First, it measured treatment readiness among offenders who entered the Prevention of Recidivism program. This is a prison-based rehabilitation program in the Netherlands that aims to lower re-offending rates among offenders with a prison sentence of at least for months and that is carried out during the final months of incarceration. Second, the study evaluated whether treatment readiness was associated with treatment participation. Third, the study examined whether treatment readiness measured with a validated instrument predicted treatment participation above and beyond a clinical assessment of treatment readiness, currently used as a criterion to include offenders in rehabilitation programs. To address these aims, data were used from the fourth wave of a research project studying the effects of imprisonment on the life of detainees in the Netherlands. Results indicated that treatment readiness as measured with a validated instrument was a significant predictor of treatment participation. Also, the current study showed that treatment readiness measured with a validated instrument improved the prediction of treatment participation above and beyond a clinical assessment of treatment readiness. Outcomes were discussed in light of study limitations and implications.