Bulk lanthanum oxide and lanthanum oxide supported on carbon nanofibers were studied as solid base catalysts. The impact of synthesis parameters on the supported catalysts, deposition method, aging, washing, and heat treatment temperature were investigated and evaluated by studying the catalytic activities in the self-condensation reaction of acetone to diacetone alcohol. The initial activity of the carbon nanofiber (CNF) supported lanthanum oxide catalysts (48 mmolDAA h−1 gcatalyst−1, DAA=diacetone alcohol) was up by one order of magnitude compared to the activity of the bulk lanthanum oxide (4.4 mmolDAA h−1 gcatalyst−1). The most active catalyst reported here was 12 wt % La2O3/CNF, prepared from a lanthanum nitrate precursor by precipitation with ammonia, followed by washing, drying, and heating at temperatures between 600–700 °C in nitrogen to activate the catalyst. Initial enthalpies for CO2 adsorption, determined by using micro calorimetry, are approximately 155 kJ mol−1 (CO2 uptake 0–5 μmol g−1) for supported samples with 12 wt % La2O3. TEM revealed well-distributed and small (≈4 nm) lanthanum oxide particles on the support.