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Auditory masking of unwanted sounds by wanted sounds has been suggested as an approach to soundscape improvement. Anecdotal evidence exists on successful applications, for instance use of fountain sounds for masking road traffic noise in urban parks. However, basic research on auditory masking of environmental sounds is lacking. Therefore, we conducted two listening experiments on auditory masking, using binaural recordings from a city park in Stockholm exposed to traffic noise from a main road and sound from a large fountain located in the centre of the park. In Experiment 1, 12 listeners assessed the loudness of road traffic noise and fountain sounds from recordings at various distances from road, with or without the fountain turned on. In Experiment 2, the same listeners assessed loudness of manipulated sound levels of singular or combined road traffic or fountain sounds. The results of Experiment 1 showed that the fountain sound reduced the loudness of road traffic noise close to the fountain, and that the fountain sound was equally loud or louder than the road traffic noise in a region 20-30 m around the fountain. This suggests that fountain sounds may add to the quality of city park soundscape by reducing the loudness of the (presumably unwanted) traffic noise. On the other hand, results from both experiments showed that road traffic noise was harder to mask than fountain sound. Furthermore, Experiment 2 showed that partial loudness of both sources was considerably less than expected from a model of energetic masking. This suggests that informational masking due to target-masker similarity may reduce the overall masking effect of environmental sounds., QC 20140923