The domestic microwave oven is now commonplace in the home and is often used to reheat chilled ready meals. Previous studies by the authors have demonstrated that the power output of such ovens can drop significantly within the first few minutes of use. This study has demonstrated that this power output reduction can have a significant affect on the expected final food temperatures after heating. A reproducible procedure was used to carry out heating tests of simulated chilled ready meals in 16 different models of domestic microwave ovens. All of the test loads had lower minimum measured temperatures (mean of three replicates) after heating in the hot ovens (ovens that had been in use for 15 min prior to the heating test) compared to when they were heated for the same time in the cold ovens (ovens that had not been used for >= 6 h). Minimum measured temperatures fell from a mean of 70.3 degrees C in the cold ovens to mean of 61.3 degrees C (range 50.3-67.6 degrees C) in the hot ovens. In 12 out of the 16 ovens (75%) these reductions in minimum temperature were statistically significant (P <= 0.05). There is an increased risk in terms of the potential for survival of bacteria such as Listeria monocytogenes when reheating food products to these lower temperatures. It was evident that at the minimum temperatures attained in the food simulant test loads heated in all the hot microwave ovens, the heat treatment would be insufficient to produce a 106 reduction of L. monocytogenes even allowing for an acceptable standing time. (C) 2007 Elsevier Ltd. All rights reserved.