The initial polymerization reactions in particle forming Ar/He/C2H2 plasmas are studied using molecular beam mass spectrometry (MBMS). The measured mass spectra are disentangled and quantified with the help of Bayesian probability theory. This approach uses the measured mass spectra and the cracking patterns (CPs) of the species that are formed in the plasma as the main input parameter. The CPs are either taken from calibration measurements or the NIST database or estimated based on a comparison to CPs of similar molecules. These estimated CPs are then modified by Bayesian analysis to fit the measured data. The CPs of C6H2, C6H4, and C8H2, which are not available in the NIST database, are determined in this way and can serve as good estimation until precise data is published. The temporal evolution after plasma ignition of the densities of in total 22 species (hydrocarbons, noble gases, and impurities) are quantified and expressed as partial pressures. The most abundant products in our plasma are C4H2 and C6H2 molecules with maximum partial pressures of 0.1 and 0.013 Pa, respectively. Our quantitative data can be used to validate plasma chemistry models. First comparison is made to a plasma chemistry model of similar C2H2 plasma already available in the literature. The comparison indicates that dissociative electron attachment to C2nH2 (n > 1) molecules is a dominant source of negative ions in C2H2 plasmas. Additionally, the C2H4 has been identified as a precursor for CnH4 molecules.