Reliable methods for the quantification of microplastics are essential in order to understand the extent and risk of exposure to humans and the environment. Pyrolysis Gas Chromatography/Mass Spectrometry (Pyr-GC/MS) is rapidly becoming a popular method of choice for environmental microplastics analysis due to its potential to accurately identify and quantify microplastics. Pyr-GC/MS and other thermogravimetric methods rely on virgin plastic standards to quantify potentially weathered microplastics in environmental samples, which may affect the accuracy of any measurement. This study assessed the influence of accelerated artificial weathering (filtered xenon-arc) on isotactic polypropylene microplastics to examine whether photo-oxidation affects their quantification. Two different shapes of polypropylene; beads (oval shape, ~5 mm, exposure time series: 0-80 days) and particles (irregular shaped, 250-500 and 500-1000 , exposure time series: 0-37 days) were examined. The photooxidation of the microplastic samples was confirmed via Fourier-transform infrared spectroscopy with Attenuated Total Reflection (FTIR-ATR), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). The rate of surface oxidation occurred faster for particles compared to beads, possibly due to their higher surface area. The signal of the pyrolysis product, 2,4-dimethyl-1-heptene, relative to mass of polypropylene was reduced through weathering. Polypropylene microplastic weathered under harsh conditions may be underestimated by 42% (particles, carbonyl index: 18) to 49% (beads, carbonyl index: 30) when quantified by Pyr-GC/MS and using virgin calibration standards. Pyrolysis at a lower pyrolysis temperature (350 oC) identified six degradation specific markers (oxidation products) that increased in concentration with weathering. Further research needs to address comparisons between virgin and weathered microplastics to avoid underestimation of microplastic concentrations in future quantification studies. They also need to stablish whether the degradation extent of weathered microplastics, coming from specific types of environments or ecosystems, are reaching high degradation extents similar to the reported ones that can cause underestimations.