La-doping mechanisms and thermoelectric properties of Sr3Ti2O7 Ruddlesden-Popper (RP) ceramics sintered under air and flowing 5% H-2 at 1773 K for 6 h have been investigated. Changes in lattice parameters and conductivity revealed a limited interstitial anion mechanism (similar to 1 at.%) based on La3+ + 1/2O(2-)-> Sr2+, which resulted in insulating samples when processed in air. In contrast, electronic donor-doping (La3+ + e(-) Sr2+) and oxygen loss [O2--> 1/2 O-2 (g) + 2 e(-)] are the dominant mechanism(s) in 5% H-2-sintered ceramics with a solution limit of similar to 5 at.%. The increased solubility limit is attributed to the formation of Ti3+ during reduction, which compensates for the extra positive charge associated with La on the A-site and also to the occurrence of oxygen loss due to the reducing conditions. For 5% H-2-sintered samples, an insulating surface layer was formed associated with SrO volatilization and oxygen uptake (during cooling) from the sintering. Unless removed, the insulating layer masked the conductive nature of the ceramics. In the bulk, significantly higher power factors were obtained for ceramics that were phase mixtures containing highly conductive perovskite-based (Sr,La)TiO3- (ST). This highlights the superior power factor properties of reduced perovskite-type ST phases compared to reduced RP-type Sr3Ti2O7 and serves as a precaution for the need to identify low levels of highly conducting perovskite phases when exploring rare-earth doping mechanisms in RP-type phases.