In this paper experimental information on the retention time distribution (RTD) of solid particles in a high-aspect-ratio vessel, stirred by three equally spaced Rushton turbines, is presented. The relevant data were obtained by a special technique named twin system approach (TSA) that greatly simplifies the handling of particle-laden streams and is therefore particularly suited for investigating particle RTD in flow systems. The technique fundamentals are first summarized, together with the data analysis procedure. This lastly requires a numerical deconvolution operation that is easily performed with the help of Z-transforms. Two different approaches for excluding the spurious contributions of the external piping required for the experimentation are tested and discussed. Particle tracing was performed by an effective particle-coating/optical-detection technique that allows particles recovery and reuse after each experimental run. The RTD data obtained indicate that a cascade of ideally mixed tanks with backflow results into very good agreement with experiment, with practically any number of tanks in series but one, provided that the backflow rate parameter is chosen accordingly. In all cases, the recirculation is large enough for the resulting flow model to be quite close to a single perfectly stirred vessel. (C) 2004 Elsevier Ltd. All rights reserved.