In this paper, we present our research on the acquisitionof gesture information for the study of the expressivenessin guitar performances. For that purpose, we design a sensor system which is able to gather the movements from lefthand fingers. Our effort is focused on a design that is (1)non-intrusive to the performer and (2) able to detect fromstrong movements of the left hand to subtle movements ofthe fingers. The proposed system is based on capacitive sensors mounted on the fingerboard of the guitar. We presentthe setup of the sensor system and analyze its response toseveral finger movements.