•The divertor DRGA for ITER will measure neutral gas composition in the pumping ducts during plasma.•System must respond in timescales relevant to compositional changes in the divertor plasma.•It is shown that times can vary from 1 to 6s for fuel (H2, D2, T2) up to 50s for He (fusion reaction ash).•It is shown that present design delivers ∼ 1s response even via an 8m long sampling pipe sampling.•Response time validated with VacTran® over anticipated the 0.1–10Pa pressure range in the ducts. One of the ITER diagnostics having reached an advanced design stage is a diagnostic RGA for the divertor, i.e. residual gas analysis system for the ITER divertor, which is intended to sample the divertor pumping duct region during the plasma pulse and to have a response time compatible with plasma particle and impurity lifetimes in the divertor region. Main emphasis is placed on helium (He) concentration in the ducts, as well as the relative concentration between the hydrogen isotopes (mainly in the form of diatomic molecules of H, D, and T). Measurement of the concentration of radiative gases, such as neon (Ne) and nitrogen (N2), is also intended. Numerical modeling of the gas flow from the sampled region to the cluster of analysis sensors, through a long (∼8m long, ∼110mm diameter) sampling pipe originating from a pressure reducing orifice, confirm that the desired response time (∼1s for He or D2) is achieved with the present design.