In this work a robust, non-invasive and wearable micro-fluidic system was developed and employed to analyse pH of sweat in real time during exercise. The device is incorporated in an optical detection platform designed to provide real-time information on sweat composition. The device has been tested by monitoring the pH of sweat during 55 minutes of cycling activity. During these trials, the data obtained by the micro-fluidic system was compared to pH measurements obtained in parallel studies with a conventional electrochemical sensor.