Cell-penetrating peptides (CPPs) are promising as drug carriers. However, because their cellular uptake mainly involves endocytic mechanisms, endosomal trapping of the carrier (and drug) remains a high barrier for biomedical applications. The viral fusion mimic GALA, a pH-triggered CPP, takes advantage of the decreasing pH during endosome maturation to selectively attack endosomal membranes. Below pH 6, the sequence folds into a helix and can disrupt membranes. Previous studies have shown that the presence of hydrophobic interfaces strongly suppresses the reversibility of the GALA folding process, and hence the bioactivity of GALA after escape from the endosome is a crucial question for the design of safe, GALA-based drugs. In this study, we show GALA remains pH-responsive when interacting with a lipid membrane and can be 'switched' back to its inactive state, even after incorporation into lipid membranes.