Hydrosols are hydrodistillation products of aromatic plants. They contain less than 1g/L of dispersed essential oils giving organoleptic properties. Hydrosols are subjected to microbial proliferation. Reasons for spoilage have to be found in the nature of substrates supporting growth and of microbiological contaminants. The composition in essential oils and the microbiota of 22 hydrosol samples of Citrus aurantium L. ssp. amara L. (orange blossom), Rosa damascena Miller (rose D.), and Rosa centifolia L. (rose C.) flowers were analyzed to determine the factors responsible for decay. The median concentrations in essential oils were 677mg/L for orange blossom hydrosols, 205mg/L for rose D. hydrosols, and 116mg/L for rose C. hydrosols. The dry matter content of these hydrosols varied between 4.0mg/L and 702mg/L, and the carbohydrate content varied between 0.21mg/L and 0.38mg/L. These non-volatile compounds were likely carried over during distillation by a priming and foaming effect, and could be used as nutrients by microorganisms. A microbial proliferation at ambient temperature and also at 5°C has been observed in all studied hydrosols when stored in a non-sterile container. In contaminated hydrosols, maximal counts were about 7log10CFU/mL, while the French pharmacopeia recommends a maximal total bacterial count of 2log10CFU/mL. Neither yeast nor mold was detected. The isolated microbial population was composed of environmental Gram-negative bacteria, arranged in four major genera: Pseudomonas sp., Burkholderia cepacia complex, and presumably two new genera belonging to Acetobacteraceae and Rhodospirillaceae. Among those bacteria, Burkholderia vietnamiensis and Novosphingobium capsulatum were able to metabolize volatile compounds, such as geraniol to produce 6-methyl-5-hepten-2-one or geranic acid, or phenylethyl acetate to produce 2-phenylethanol. EO concentrations in hydrosols or cold storage are not sufficient to insure microbiological stability. Additional hurdles such as chemical preservatives or aseptic packaging will be necessary to insure microbial stability.