A study about the temporal variation of organic emissions from a modern wood log fired masonry heater was carried out with different gas analysis techniques: single-photon ionisation time-of-flight mass spectrometry (SPI-TOFMS) for real-time analysis of volatile (VOC), intermediate-volatile (IVOC) and semi-volatile organic compounds (SVOC), and a gas analyser system for gaseous components CO2, CO, NOx and organic gaseous carbon (OGC, quantified by flame ionisation detector). The emissions of three in Europe common types of firewood (beech, birch and spruce) were investigated by combustion of six consecutive batches of 2.5 kg each over 4 h. Batchwise emissions and temporal variations during combustion were discussed. Emission factors over the whole combustion cycle for OGC, VOC and IVOC were right up to one order of magnitude lower than in many previous studies due to latest improvements of air staging technology in wood log fired masonry heaters, whereas CO and NOx remained comparable. Regarding each combustion experiment, more than 50% of the total intensity of the mass spectra occurred during the combustion of the first two batches. Moreover, the molecular signatures of burning phases ('ignition', 'stable combustion' and 'ember') were examined by using non-negative matrix factorisation (NMF) and principal component analysis (PCA) in sequence. Marker substances for wood or biomass combustion, such as phenolic species or furan derivatives, exhibited highest relative abundance during 'stable combustion', whereas 'ember' is distinctly characterised by polyunsaturated hydrocarbons, such as benzene or naphthalene, through pyrosynthesis; in 'ignition', secondary decomposition products dominated. Nevertheless, highest quantitative emissions always occurred during 'ignition' at the beginning of each batch, followed by the phases 'ember' and 'stable combustion'. (C) 2016 Elsevier Ltd. All rights reserved.