Two organic fouling samples obtained from downstream the cracking oven (DS) and from upstream the hot zone (US) of a steam cracker facility were characterized. For this purpose, a simultaneous thermal analyzer coupled to a photoionization mass spectrometer (STA-PI-MS) and a thermal desorption/pyrolysis gas chromatograph (TD/Py-GC-EI-MS) were used. Mass loss and differential scanning calorimetry information revealed the degradation of the materials beginning at 130 degrees C with two distinct maxima for US and one for DS (230-330 degrees C) as well as broad signals (330-500 degrees C) for both. Structural motives of different polymeric-like structures were assigned based on PI-MS of the effluent and separately conducted TD/Py-GC-EI-MS. The advantage of soft photoionization over hard ionization techniques such as electron ionization is the considerable reduction of fragmentation, yielding higher abundancies of molecular ions. Thus, even though complex samples are studied, evolving constituents can often be easily tracked in a time-resolved manner (1 Hz). While single photon ionization (SPI, 118 nm = 10.5 eV) ionizes most organic molecules, resonance-enhanced multiphoton ionization (REMPI, 2 x 266 nm = 9.4 eV) selectively addresses aromatic species. Differentiation of polymeric-like structures was achieved by exploiting this selectivity (SPI vs REMPI) and comparison of molecular patterns with GC-EI-MS data, which supports the identification of compounds by providing fragmentation patterns and chemical information based on retention time. US shows high inorganic content (similar to 50%) and more diversity in its organic part, as indicated by four types of patterns: polyethylene-like, Diels-Alder-like polythioether/polysulfide-like, and polystyrene-like motives. In contrast, DS exhibits almost only signals of Diels-Alder-like and polystyrene-like structures and contains a less inorganic material (similar to 23%). Additionally, first attempts to quantify the Diels-Alder content by STA-SPI-MS were successfully conducted.