Abstract: A method for monitoring surface phenomena includes measuring a first surface plasmon resonance angle value (?SPR,REF) of a sample region (REG1), measuring a first critical angle value (?TIR,REF) of the sample region (REG1), causing a change of surface concentration (cM1,SRF) of an analyte (M1) at the sample region (REG1), changing the bulk composition at the sample region (REG1), measuring a second surface plasmon resonance angle value (?SPR(t)) of the sample region (REG1), measuring a second critical angle value (?TIR(t)) of the sample region (REG1), and determining an indicator value (?AUX(t)) indicative of the change of the surface concentration (cM1,SRF), wherein the indicator value (?AUX(t)) is determined from the second surface plasmon resonance angle value (?SPR(t)) by compensating an effect of the bulk composition, and wherein the magnitude (?COMP) of said effect is determined by using the second critical angle value (?TIR(t)).