Fiber Bragg based – fiber optic sensors were applied in operando to monitor the temperature of illuminated plasmonic catalysts at various depths inside the catalyst bed during light-driven CO2 hydrogenation. Multipoint temperature measurements showed that single-sided illumination induced a pronounced vertical temperature gradient, which remained stable throughout the reaction. This behaviour was observed in two light driven reactions: the exothermic Sabatier reaction catalysed by Ru/Al2O3 and the endothermic reverse water gas shift reaction catalysed by Au/TiO2. The temperature gradient, attributed to a combination of limited light penetration depth and poor thermal conductivity of the catalyst bed, must be taken into account in kinetic studies. Metal loading and gas composition had a strong influence on the temperature gradient, while gas flow rate and reaction heat had a negligible effect. For catalyst temperatures up to 250˚ C, radiative heat loss accounted for approximately 15 % of the incident light power. Our study demonstrates that accurate in operando temperature monitoring at multiple positions inside the catalyst bed is essential to distinguish between thermal and non-thermal contributors in plasmon catalysis.