This paper investigates the Overbidding phenomenon in first-price independent private values auctions. Experiments were conducted to investigate the hypothesis that overbidding may be driven by misperception about either the probability of winning the auction or the derivative of this probability. This paper also investigates possible causes of this misperception. The experimental outcomes confirm the presence of the overbidding phenomenon and indicate that the forecasting error of the chance of winning the auction is an important factor, although it can not fully account for the observed bias. The results also indicate that misperception of probabilities’s derivatives is not correlated with overbidding.