The concentration of dissolved oxygen is important for the life of all aerobic marine organisms because they consume oxygen during respiration. During periods of stratified water column, especially in late summer or early autumn, episodes of hypoxia and anoxia may occur. Long-term measurements of dissolved oxygen concentrations in the bottom layer of the sampling station in the central part of the Gulf of Trieste show significant fluctuations on both annual and long-term scales. During the period 1990-2010, average dissolved oxygen concentrations increased significantly, then the trend reversed and lower values were measured again. Nevertheless, seasonal periods of persistent bottom oxygen deficiency, which were common in the past and often had negative impacts on benthic communities, have not been recorded since 2002.