Since the work by Radetzski et al. (1994) on the use of oxalic acid for controlling varroosis, several reports have been published proving the efficacy and tolerability of this organic acid when applied to honeybees (Apis mellifera L.) by spraying or trickling. Tests were carried out by spraying water-diluted oxalic acid (Nanetti et al., 1995) or trickling a solution of oxalic acid, water and sugar (Imdorf et al., 1997;Mutinelli et al., 1997;Nanetti and Stradi, 1997) into colonies during broodless periods. Oxalic acid, water and sugar solutions and the water-diluted oxalic acid (Highes et al., 1999) have occasionally been considered responsible for honeybee losses after treatment, and no data are currently available on their long-term stability. Since such solutions are widely used by beekeepers for controlling varroosis in broodless periods, our aim was to evaluate the characteristics and stability of a homemade oxalic acid, water and sugar solution in various storage conditions, in order to detect modifications of the active ingredients possibly related to toxicity towards honeybees. The homemade oxalic acid/water/sugar solution (OAWS) prepared in the laboratory for the present study is the same as that used for treating beehives against varroosis. It is composed of 100 g oxalic acid dihydrate (Sigma), 1000 g commercial sugar and 1 000 ml drinkable water. The characteristics of water were as follows: pH 7.5, solids 320 mg . L -1 , hardness 27.1 °F, chloride 8.0 mg . L -1 , nitrate 17.5 mg . L -1 , sulphate 21.5 mg . L -1 , iron 5 µg . L -1 . The density of the final solution is 1.236 g . L -1 and the concentration of oxalic acid is 4.2% (0.467 Mol).This solution was stored in 10-ml tubes at various conditions: -20 °C, +4 °C and room temperature (RT) in the dark and in the light. The solution was tested for the following parameters immediately after its preparation: oxalic acid, colour, pH, hydroxymethylfurfural and sugars. The same parameters were determined at 3, 7, 15 and 30 days, and at 2, 3, 4, 5, 6, 7, 8 and 16 months after preparation for each storage condition. Oxalic acid was determined using a commercially available kit (Oxalate Sigma Diagnostics kit, Cat. N. 591-D) adapted for honey (Mutinelli et al., 1997). The colour of the OAWS solution was examined by means of a 2000 Comparator (Lovibond) used for honey colour determination. Hydroxymethylfurfural (HMF) was determined using reverse-phase HPLC equipped with UV detection (Jeuring et al., 1980). Sugars were determined by an HPLC method using a pulsed amperometric detection. This study demonstrated that the concentration of oxalic acid and pH of an OAWS solution topically applied for varroosis control remains constant in all tested storage conditions and over a long period of time, as opposed to changes affecting other parameters (Tab. I). Colour changes observed in samples stored at RT were related to the numerous possible condensation reactions (Maillard reaction), which produce many polycyclic compounds absorbing light in the visible region. The ...