[en] The frustration parameter t^'/t has been estimated for the spin-liquid system κ-ET₂Cu₂(CN)₃ at ambient pressure using the previously reported Shubnikov-de Haas data. Using data for κ-ET₂Cu(NCS)₂ as a function of pressure and for κ-ET₂Cu₂(CN)₃ in its pressure-induced metallic phase, the ratio has been estimated in several different ways, with fully consistent results. A final value of 0.95(3) is obtained, indicating the very strong degree of frustration in this system and fully compatible with its observed spin-liquid properties.