Bias stress instability has been investigated in printed p-channel organic thin film transistors. The observed instability is related to two mechanisms: one, dominating at low T and causing “mobile ions” like threshold voltage variations is probably due to creation/annihilation of acceptor-like states; the second one, causing charge-trapping like instability, dominates at high T. High drain voltage bias stress experiments, inducing device self-heating, present threshold voltage variations, suggest a channel temperature rise ranging from 50 to 60 C. The results point out the role of self-heating on the bias-stress instability, which is related to a combination of bias and temperature conditions.