Power compensation differential scanning calorimetry (DSC) has been employed to detect and analyse precipitation reactions in an Al-1.3Mg-0.4Mn and an Al-1.3Mg-0.4Mn-0.07Cu alloy in which very small amounts of precipitate, less than 0.3at.%, are expected to form. Due to the very small heat effects, baseline instability and drift significantly interfere with the measurements. After repeated experiments and careful baseline correction it is demonstrated that in the Cu containing alloy, ageing at 170degreesC causes the appearance of two endothermic effects: for 2 days ageing a small dissolution effect appears at about 230degreesC, whilst for 7 and 21 days ageing a dissolution effect peaking appears at about 300degreesC. The temperature range of the latter is consistent with S phase dissolution. (C) 2003 Elsevier B.V. All rights reserved.