Abstract: An arrangement for determining concentration of at least two sample components in solution of at least three components comprises a refractometer as a first instrument for measuring refractive index data as a first quantity of the first sample component. In addition the arrangement comprises a second physical quantity measuring device for measuring second physical quantity as a second quantity data of the second sample component, such as a device for measuring conductivity. The second physical quantity is advantageously essentially independent on said refractive index, but is more strongly dependent on at least concentration of at least one second sample component of said solution. Further the arrangement comprises a data processing unit for determining said concentration of at least two sample components by using said refractive index data and second quantity data in an additive way after a variable substitution performed by said data processing unit on the refractive index data.