The retention behavior of HfO2-based resistive switching memory cells (RRAM) is characterized as a function of Al doping concentration, which was previously reported to be a viable method for the improvement of the switching uniformity. While the low resistance state (LRS) does not exhibit any major variation up to 106 s for all the tested devices, two retention loss mechanisms can be identified for the high resistance state (HRS). The main HRS trend follows a temperature-activated gradual decrease of the resistance, which also depends on the doping concentration. In addition, tail bits of the population distribution show a very fast retention loss process that strongly depends on the doping concentration.