Plastic debris has become ubiquitous in terrestrial and marine environments, and its consequent degradation into microplastics has not been yet well understood. Although there are no standardized analytical methods, recent advancements have identified and quantified microplastics in environmental samples using Pyrolysis Gas Chromatography/Mass Spectrometry (Pyr-GC/MS). To assess the reliability of Pyr-GC/MS for measuring microplastics in environmental samples, studies have used virgin polymer standards to correct for matrix effects and to select markers specific to the polymers investigated. However, little is known about how natural weathering processes could possibly impact inherent properties of microplastics, and consequently their quantitative analysis using techniques like Pyr-GC/MS for environmental samples. This research aims to add the understanding of weathered microplastic properties and how they could possibly cause variability in quantitation due to different properties in comparison to the original standards used for quantitation. Based on this, the objectives of this study were therefore to perform accelerated weathering of isotactic polypropylene microplastics of two different forms (i.e. beads ( 5 mm) and irregular shaped particles (250-500 μm, 500-1000 μm)) via photo-oxidization using accelerated laboratory weathering, and examine whether this impacts their quantitative estimation using Pyr-GC/MS techniques. The progressive weathering and degradation until 80 days (beads) and 37 days (particles) of beads and particles respectively, was confirmed via techniques including Fourier-transform Infrared - Attenuated Total Reflection (FTIR-ATR) spectroscopy, scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). Comparison of pyrograms from Pyr-GC/MS of the progressive weathering was used to identify any changes to the quantification of virgin and aged MPs, specifically the pyrolysis product, 2,4-dimethyl-1- heptene. Quantitative changes were significant in terms of peak areas and suggest that quantitation using Pyr-GC/MS can be affected by weathering processes.