A study of the meson-baryon interaction in the S = −1 sector is performed, employing a chiral SU(3) Lagrangian up to next-to-leading order (NLO) and implementing unitarization in coupled channels. The model is constrained by a large set of experimental data, paying special attention to processes that are sensitive to the NLO contributions, such as the K− p → K+−, K00 reactions. The consideration of additional cross sections in single isospin channels, K− p → η , η , has been found to provide NLO low-energy constants of rather similar size. The stability of these constants has also been tested by the inclusion of explicit resonant terms. Predictions for new isospin filtering processes, like the I = 1 K0 L p → K+0 reaction that could be measured at the proposed secondary K0 L beam at Jlab, or the weak decay of the b into a J/ and different meson-baryon pairs in I = 0, available at LHCb, are presented. The measurement of such reactions would put valuable constraints on the chiral models describing the S = −1 meson-baryon interaction.