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8 pags., 6 figs. ; Er doping of TiO colloidal nanocrystals enhances their performance for photo-induced applications. Such doping is known to delay the anatase to rutile transformation under thermal treatment; nonetheless relevant information on the Er light emission and location within the TiO structures is still incomplete. Er photoluminescence emission both in the visible (upconverted) and infrared photoluminescence is used for the first time to probe the ions location within the different TiO structures. The results show that Er ions in the as-prepared xerogels are not embedded in the anatase crystallites, and only upon thermal treatment Er diffusion is induced into crystal interstitial positions to form a solid solution. At higher temperatures rutile is formed inducing Er segregation and giving rise to the formation of pyrochlore (ErTiO), as shown by a distinct emission in the infrared spectrum due to the Er ions located within the pyrochlore compound. Although pyrochlore is usually a high temperature phase, analysis of the photoluminescence allows its labeling at temperatures as low as 600–700 °C for small Er concentrations (1 mol %). Increasing Er concentration promotes its enrichment at the nanocrystallites surface accomplished by the anatase-to-rutile phase transformation, suggesting that Er ions control the TiO nanocrystals surface properties. ; This work was supported by the Ministerio de Economía y Competitividad (MINECO, Government of Spain) under Grants MINECO/FEDER TEC2015-69916-C2-1-R and MAT2015-65356-C3-1-R. IC and MB acknowledge the grants JAE-Pre-201100578 and JAE-Pre-083, respectively. ; Peer Reviewed