A SPR sensing method comprising the steps of: providing a SPR sensor comprising a SPR supporting sensor surface and contacting a sample to be analysed with the sensor surface. At least one resonance condition at said SPR supporting sensor surface is monitored by illuminating the sensor surface with an SPR exciting test light beam and sensing the reflected or transmitted test light beam. Additionally, the sensor surface is illuminated with a reference light beam under conditions selected so as not to excite SPR at said sensor surface and sensing the intensity of the reflected or transmitted reference light beam. At least one property of the reflected or transmitted test light beam is determined taking into account the sensed intensity of the reflected or transmitted reference light beam.