The conformational behavior of poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) brushes with different chain density in the presence of large benzyltributylammonium chloride (BTBAC) ions has been studied by a Quartz Crystal Microbalance with Dissipation (QCM-D) and Scanning Force Microscopy. Dense brushes do not collapse in the presence of BTBAC solutions of increasing ionic strength, contrary to what is observed in the presence of NaCl. Brush collapse can be observed for low-ionic-strength solutions of BTBAC when the brush density has been reduced. These phenomena can be explained by considering the Hofmeister series as well as ion size and free space in the brush.