Nanoparticle (NP) sensors are prepared by Langmuir-Blodgett deposition of colloid Fe 2 O 3 or CoFe 2 O 4 NPs onto alumina substrates. The thickness of the deposit is 1, 2, 4 or 7 NP monolayers (ML). NP arrays are analysed by GIXRD, SEM, SAXS, DLS and XANES. The average NP diameters are 6.4 nm and 7.6 nm for Fe 2 O 3 and CoFe 2 O 4 , respectively. The response current of the sensor is measured at ≈ 400 °C in the mixtures of dry air with the monitored CO and NO 2 gases. The best relative response is 38 for 500 ppb of NO 2 . The resistance of NP arrays was studied between 250 and 500 °C. With 2 ML Fe 2 O 3 deposit the sheet resistance decreases with increasing temperature from 10 11 to 10 9 Ω. Activation energy of the semiconductor type conductivity is 0.75 eV. High gas monitoring sensitivity of NP sensors is due to the big effective surface of Fe 2 O 3 and CoFe 2 O 4 NP arrays.