Excipients used in lyophilized protein drug products are often selected by a trial-and-error method, in part, because the analytical methods used to detect protein-excipient interactions in lyophilized solids are limited. In this study, photolytic labeling was used to probe interactions between salmon calcitonin (sCT) and excipients in lyophilized solids. Two diazirine-derived photo-excipients, photo-leucine (pLeu) and photo-glucosamine (pGlcN), were incorporated into lyophilized solids containing sCT, together with an unlabeled excipient (sucrose or histidine) at prelyophilization pH values from 6 to 9.9. Commercially available pLeu was selected as an ionizable photo-excipient and amino acid analog, while pGlcN was synthesized as an analog of sugar-based excipients. Photolytic labeling was induced by exposing the solids to UV light (365 nm, 30-60 min), and the resulting products were identified and quantified with liquid-chromatography mass spectrometry. The distribution of photo-reaction products was affected by the photoreactive reagent used, the type of unlabeled excipient, and the solution pH before lyophilization. When other components of the solid were identical, the extent and sites of labeling on sCT were different for pGlcN and pLeu. The results suggest that ionizable and nonionizable excipients interact differently with sCT in lyophilized solids and that photo-excipients can be used to map these interactions. Copyright © 2020 American Pharmacists Association®. Published by Elsevier Inc. All rights reserved.