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Solid composite polymer electrolytes (CPEs) are complex mixtures of ceramics, polymers, and lithium salts, where the interfaces between the different phases play an important role for stability, conductivity, and compatibility with electrode materials. In this study, two interfacial phenomena of CPEs consisting of lithium lanthanum zirconium oxide (LLZO) ceramic fillers in poly(trimethylene carbonate) (PTMC) with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt are studied. First, the LLZO-polymer electrolyte interfaces are investigated. Second, the stability of this CPE material vs a Li-metal electrode is explored, by employing soft X-ray photoelectron spectroscopy (PES) in combination with in situ deposition of Li. Three different LLZO loadings in PTMC are investigated: 30, 50, and 70 wt %. The concentration of LiTFSI follows that of the particle concentration at the surface of the samples, where the CPE with 50 wt % bulk content of LLZO exhibits the highest surface concentrations of both salt and ceramic. This shows an affinity for the salt at the LLZO surface. Furthermore, the stability of the CPEs against Li is studied after in situ Li deposition and shows that PTMC can decompose, potentially forming polypropylene at the CPE|Li interface, with the CPE at 50 wt % of LLZO showing the most pronounced PTMC and TFSI breakdown. This is in agreement with the observed properties for the polymer-ceramic interfaces and highlights the decisive role of LiTFSI accumulation on the surface of the ceramic particles, both for ionic transport and chemical stability.