A study of organic compounds imparting sweet and buttery odor problems in the Llobregat River (northeast Spain) and in treated water was conducted. Solid-phase microextraction (SPME), gas chromatography-olfactometry (GCO) and flavor profile analysis (FPA) were used as analytical methodologies to identify the compound responsible for odor incidents. Diacetyl (2,3-butanedione) with a concentration range of 0.90-26 µg/L in river water samples entering the water treatment plant (WTP) was identified as the compound causing the odor events. Flavor profile analysis establishes 0.05 µg/L as its odor threshold concentration (OTC) in water, with an odor recognition concentration of 0.20 µg/L. Includes 19 references, tables, figures.