A method for determining the concentration of an analyte in tissues. The method involves compensating for a change in the value of an optical property of the tissues, such as, for example, the scattering coefficient, resulting from a change in the hydration status of the tissues. The method comprises the steps of: (a) measuring at least one optical property of a tissue sample at at least one wavelength at an initial time; (b) calculating the absorption coefficient and the scattering coefficient of the tissue sample at the initial time; (c) repeating the measurement of the at least one optical property of the tissue sample at at least a later time at the at least one wavelength; (d) calculating the absorption coefficient and scattering coefficient of the tissue sample at at least the later time; (e) calculating the change in the value of the absorption coefficient at the at least one wavelength to indicate the change in the water content of the tissue sample and the change in the value of the scattering coefficient to indicate both the change in the water content of the tissue sample and the change in concentration of an analyte in the tissue sample; (f) correcting the value of the scattering coefficient to account for the effect of the change in the water content of the tissue sample; and (g) calculating the concentration of the analyte by means of the corrected value of the scattering coefficient.