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Phosphate treatments for conservation of stone have provided so far encouraging results in laboratory studies, as they exhibit good effectiveness, compatibility and durability to accelerated weathering tests. However, limited data are available about their long-term performance in real environment. Here, a systematic evaluation of phosphate consolidants after prolonged exposure in the field is reported for the first time. Naturally weathered marble specimens and a XVII century marble sculpture located in the Park of the Royal Palace in Versailles were treated by various formulations of the phosphate consolidant. Their conservation state was assessed before treatment by non-destructive methods (ultrasounds, color measurement, water absorption) and, in the case of the specimens, also by slightly destructive tests (SEM, FT-IR, MIP, IC). The conservation state of the specimens and the sculpture was further assessed right after treatment and then periodically monitored during exposure in the Versailles Park. Characterization after field exposure demonstrated that some formulations of the phosphate treatment are able to slow down marble deterioration, although it was not completely inhibited. Limited alterations in water absorption and aesthetic appearance confirmed the general compatibility of the phosphate treatment.