A method and a device are proposed for determining the type of fluid in a fluid mass in an object. X-ray attenuation data is supplied from one or a plurality of X-ray recordings of an object area including the fluid mass in the object, which were acquired with at least two different X-ray spectra or detector weightings. The X-ray attenuation data is used to determine values for effective atomic number and density for the fluid mass and average these to obtain a mean value for effective atomic number and density for the fluid mass. Comparison data is also supplied, which indicates fluctuation ranges for combinations of effective atomic number and density for different types of fluid. The mean values for effective atomic number and density of the fluid mass are compared with the comparison data to determine the fluctuation range and thereby the type of fluid, into which the two mean values fall. The method and associated device can be used to determine the type of fluid in a fluid mass in an object in a reliable and unambiguous manner.