A balloon catheter comprising a catheter (3) and a balloon (7) located at a distal end (5) of the catheter (3). A plurality of measuring electrodes (12) formed on a primary substrate (17) extend around the catheter (3) within the balloon (7). The primary substrate (17) is formed by a portion of a flexible resilient membrane (25) which also forms a secondary substrate (19). The measuring electrodes (12) are formed on the primary substrate (17) by first electrically conductive tracks (27) and second electrically conductive tracks (28) which are electrically connected to the first electrically conductive tracks (27) are simultaneously formed on the secondary substrate (19). The membrane (25) is coiled to form a roll, and the primary substrate (17) is also coiled around the rolled secondary substrate (19). The coiled primary and secondary substrates (17,19) is urged through a second lumen (20) in the catheter (3) from the proximal end (4) thereof to a radial slot (21) in the catheter communicating with the second lumen (20). The primary substrate (17) is urged through the radial slot (20) and wrapped around and bonded to the catheter (3) to form the measuring electrodes (12) as band electrodes.