The neutron dose equivalent in aircraft is measured using commercial bubble detectors, but at high altitudes there are not only neutrons but also protons. Bubble detectors have been used because they are considered not to be sensitive to protons, but this has been the subject of only a few studies. In this study, by irradiating bubble detectors with energetic protons, the detectors' responses to protons were observed. Bubbles were clearly formed by protons, but the proton sensitivities were one order of magnitude smaller than the neutron sensitivities. Thus, counts of protons can be ignored in neutron measurements in aircraft.