High acid levels, characteristic of rubber seed oil (RSO), limit RSO use in biodiesel production. The aims of this study were to determine the causes of these high acid levels by investigating what affects the storage of rubber seeds and RSO had on the acid levels. Two storage conditions/methods were evaluated, one representing a proposed storage method (SM 1), the other mimicking storage conditions characteristic to the Xishuangbanna region (SM 2). Furthermore, RSO storage was evaluated by testing RSO acid levels over a 2-month period, under standard storage conditions. Seeds from SM 2 displayed increased seed pile temperatures, higher levels of Mildew infection, lower seed oil content and higher acid levels. Low seed oil content and high acid values of SM 2 were resultant of the high Mildew infection and increased seed pile temperatures. In addition, a critical value of 90% relative humidity of seed piles was identified, above which Mildew infection increased sharply. Storage of crude RSO resulted in increased acid values. This data shows that in order to reduce high acid values, seed pile temperature, humidity and Mildew infections need to be kept to a minimum, as well as the storage time of the seeds and the RSO. (C) 2011 Elsevier Ltd. All rights reserved.