The effects of the polymorphism of glutaric acid (GA) particles on their deliquescence properties at room temperature were studied. The polymorphic states of GA, including the metastable alpha-form and the thermodynamically stable beta-form, were characterized by Raman spectral measurements. The deliquescence of GA particles was also studied by Raman and optical observations. It was found that the alpha-form deliquesced at 85-86% relative humidity (RH), whereas some alpha-form transformed to the beta-form rather than completing full deliquescence. The beta-form absorbed water slowly at 87-89% RH, which was higher than the deliquescence relative humidity (DRH) of the alpha-form. The water-uptake process was not completed in 5 h, but the particles deliquesced quickly (<15 min) when the RH was increased to about 90%, indicating that mass transfer was delayed in the beta-form and that a higher RH of 90% was needed to cross the kinetic barrier. We showed that polymorphic transformation alters the deliquescence properties of GA particles, which might explain the discrepancies in DRH values of GA particles (83-90%) reported in the literature. This transformation should be taken into account in future laboratory experiments and aerosol thermodynamic models.