Abstract: Instigated by developing a flow injection procedure for assay of nitrosyl in concentrated H2SO4, different approaches for reliable and robust online dilution in FIA were evaluated. These comprised the application of mixing tees in conjunction with mixing coils (including knotted reactors) of different internal diameter, zone sampling, the use of a mixing chamber, micro-sampling, and sample injection by pseudo-hydrodynamic injection. The individual approaches are described in detail, their advantages and disadvantages being emphasized in regard to their practical applicability. For each approach the criteria stipulated were that the procedure should allow a dilution factor of ~100, yet without excessive zone spreading, so that it, on one hand, could effectively eliminate the pronounced Schlieren effect encountered when mixing concentrated H2SO4 with an aqueous diluent, and, however, would permit the sample material to be appropriately conditioned for the ensuing chemical derivatization procedure. This most reliably could be effected by a combination of pseudo-hydrodynamic injection comprising a mixing point and the use of mixing tees in conjunction with knotted reactors of relatively large internal diameter (1.5 mm). The optimized FI-manifold was used with the Griess method for the spectrophotometric assay of nitrosyl (nitrite) in standards prepared in the matrix of concentrated H2SO4 (detection limit 0.16 mg/L NO+-N (3s)) and in practical WSA-samples.