This invention relates to procedures for the calibration of systems for continuously measuring the concentration of substances in a body fluid. The system comprises first and second sensors adapted for subcutaneous insertion and an electronic calculator unit adapted for measuring signals from the two sensors. The system is calibrated following the steps of: a) introducing the first sensor subcutaneously, b) calibrating the first sensor, c) obtaining sensor data S1(t) provided by the first sensor, d) introducing the second sensor subcutaneously, e) obtaining sensor data S2(t) provided by the second sensor, f) determining the rate of change over time δR(t)/δt, R(t) being a signal which correlates to sensor data S2(t) over time, and g) performing a calibration of the second sensor when δR(t)/δt is less than a predetermined value, said calibration of the second sensor being performed using sensor data S1(t) obtained by the first sensor.