Pyrolysis liquids from the slow pyrolysis of a german brown coal obtained at different pyrolysis temperatures were analyzed by electrospray ionization fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS) in negative ion mode. Singly charged ions in the range from m/z = 200 to 700 were observed. Assignment of molecular formulas revealed mainly oxygen containing species, which are most probably acidic compounds ionized during ESI in negative ion mode. Compounds detected were of type O-o, OoS1, OoS2 and OoN1 with oxygen numbers o = 1 ... 12 depending on the specific class. The formation or decomposition of certain compound classes upon temperature changes was investigated. It could be shown that summation of observed compound class frequencies in terms of number or intensities is not suitable to identify trends. Identification of temperature dependent behavior of certain compound classes can only be revealed by looking at individual ions, which is exemplified for carboxylic acids. (C) 2013 Elsevier Ltd. All rights reserved.