The C-POD logger is a widely used instrument for passive acoustic monitoring of harbor porpoises, but the absence of a continuous recording in this device makes it difficult to verify its performance. An alternative but more labor-intensive approach is to use a wideband sound recorder and off-line detection software. Here we compare the performance of the C-POD with that of a HF SoundTrap recorder analysed with PAMGUARD software. Seven deployments were made with C-PODs and SoundTraps in the Danish Great and Little Belts between June and November, 2015. There was a positive but generally poor correlation between PAMGUARD and C-POD detections, with the C-PODs detecting only about 21-94% of the click trains detected by PAMGUARD based on the broadband recordings. The main explanation behind this poor correspondence is likely that PAMGUARD performs classification on single clicks, whereas the C-POD classifies groups of clicks (‘trains’) collectively. Such poor correlation between two common methods can have severe implications for conclusions reached in effect and abundance studies.