Abstract Objectives To study the effectiveness of partially degraded polyethylene glycol 6000 (PEG) as a precipitant for macroprolactin. Design and methods PEG was heated to 63 °C in air for up to 20 days and its effectiveness assessed as a precipitant for sera containing normal prolactin or macroprolactin. Decomposition was studied chemically and with NMR spectroscopy. Results Thermal degradation was similar to what had occurred over several years of natural degradation. Initially PEG degraded 2–5 days caused excess precipitation of monomeric prolactin (false-positive macroprolactinemia). Samples degraded 18–20 days failed to precipitate macroprolactin, giving false negative results. Two 1H NMR peaks at 4–4.5 ppm were not detectable in undegraded PEG but were after 1 day. Their relative integral increased to 20 days. Conclusions Aging of PEG can be accelerated by heating. The suitability of PEG for use in macroprolactin assays can be assessed by the absence of peaks at 4–4.5 ppm by 1H NMR.