Formaldehyde is a widely present pollutant in both indoor and outdoor air. Due to its high reactivity and carcinogenic nature, it poses serious health risks and measurement challenges. In fact, the International Agency for Research on Cancer (IARC) has classified formaldehyde as a Group 1 human carcinogen. As a result, countries like Italy have lowered emission limits to 5 mg/Nm3. This highlights the urgent need for accurate and reliable monitoring systems. To address this, the European Committee for Standardization (CEN) created the working group TC 264 WG 40, which in 2021 published the standard CEN/TS 17638:2021. This document outlines reference methods for measuring formaldehyde in emissions from stationary sources. In this study, formaldehyde measurement techniques were evaluated through Proficiency Tests (PTs) using the 'LOOP' test bench. LOOP is a unique facility in Italy that simulates industrial emission conditions using synthetic gases. Different measurement methods were tested, particular focus was given to Method C (acetylacetone with photometric detection) and Method D (DNPH with HPLC analysis), both described in the CEN standard. Method C showed excellent temporal repeatability, while Method D required careful derivatization for consistent results. Interferents like SO2 significantly affected accuracy when Milli-Q water was used as the sampling solution, causing underestimations of 35–80%. However, an acidified solution (H2SO4 0.01 M) reduced errors to 5–30%. These results support the development of improved protocols and confirm the importance of PTs in validating reliable emission monitoring systems for regulatory and environmental protection.