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The ISO standard 22197-1:2016 used for the evaluation of the photocatalytic nitric oxide removal has a main drawback, which allows only the decrease of nitric oxide to be determined specifically. The remaining amount, expressed as “NO₂”, is considered as a sum of HNO₃, HONO NO₂, and other nitrogen-containing species, which can be potentially formed during the photocatalytic reaction. Therefore, we developed a new methodology combining our custom-made analyzers, which can accurately determine the true NO₂ and HONO species, with the conventional NO one. Their function was validated via a photocatalytic experiment in which 100 ppbv of either NO or NO₂ dispersed in air passed over (3 L min⁻¹) an Aeroxide© TiO₂ P25 surface. The gas-phase analysis was complemented with the spectrophotometric determination of nitrates <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mrow><mo>(</mo><mrow><msubsup><mrow><mi>NO</mi></mrow><mn>3</mn><mo>−</mo></msubsup></mrow><mo>)</mo></mrow><mo> </mo></mrow></semantics></math></inline-formula>and/or nitrites (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NO</mi></mrow><mn>2</mn><mo>−</mo></msubsup><mo stretchy="false">)</mo><mo> </mo></mrow></semantics></math></inline-formula>deposited on the P25 layer. Importantly, an almost perfect mass balance (94%) of the photocatalytic NOx abatement was achieved. The use of custom-made analyzers enables to obtain (i) no interference, (ii) high sensitivity, (iii) good linearity in the relevant concentration range, (iv) rapid response, and (v) long-term stability. Therefore, our approach enables to reveal the reaction complexity and is highly recommended for the photocatalytic NOx testing.