Multifunctional materials exhibiting photon upconversion show promising applications for biological imaging and sensing. In this study, we examine the solid-state upconversion emission of NaYF4:Yb,Er nanoparticles in the presence of iron oxide nanoparticles. Fe3O4 nanoparticles (6 nm) were mixed with NaYF4:Yb,Er nanoparticles (either 10 or 50 nm) in varying proportions by drying chloroform solutions of nanoparticles onto glass slides. Upconversion spectra were acquired, and a laser power-dependent emission was observed and correlated with the iron oxide content in the mixture. Changes in the lattice temperature of the upconverting particles were monitored by careful observation of the relative intensities of the 2H 11/2 and 4S3/2 →4I 15/2 transitions. The emission characteristics observed are consistent with an iron oxide-induced thermal effect that is dependent on both the laser power and the proportion of iron oxide. The results highlight that the thermal effects of mixed nanoparticle systems should be considered in the design of luminescent upconverting hybrid materials. © 2013 American Chemical Society.