The aim of this work was to assess the effect of the presence of ash on maximum temperature reached (MTR) estimation using near infrared reflectance (NIR) spectroscopy. The degree of combustion (ash produced by heating to 100, 300, 500 and 700 °C), the type (ash from Pinus halepensis and Rosmarinus officinalis), and different quantities of ash (0–20% in 2% interval) were evaluated in a soil heated at seven different temperatures (100 °C–700 °C). Results showed that the estimation of MTR on samples with ash, using partial least squares (PLS) models constructed with samples without ash, could be erroneous. Both, ash quantity and degree of combustion affected the estimation of MTR. However, using discriminant analysis, a good classification of samples (N97% correctly classified) according to the heating temperature classes (unheated, 100, 200, 300, 400, 500, 600 and 700 °C) was obtained despite the presence of ash.