Determining the viscosity of a hydrocarbon reservoir fluid that is present in a formation layer traversed by a borehole, which method involves the steps of selecting a location in the formation layer; lowering in the borehole to the location a tool that has a central conduit having an inlet, means for displacing fluids through the central conduit, and an optical fluid analyzer; making an exclusive fluid communication between the formation and the inlet of the central conduit; obtaining a spectrum of the optical density; calculating a first factor that is the maximum optical density in a predetermined short-wavelength range multipled with the length of the short-wavelength range, calculating a second factor which is the integral over the same short-wavelength range of the spectrum, subtracting the second factor from the first factor to obtain a hydrocarbon oil property; and obtaining the magnitude of the in-situ viscosity from the oil property using a relation that has been obtained by fitting a curve through previously obtained data points having the measured magnitude of the actual viscosity as a function of the oil property.