Lead-free (1 - y)(Bi1/2Na1/2TiO3-xBi(1/2)K(1/2)TiO(3))-yK(0.5)Na(0.5)NbO(3) [(1 - y)(BNT-xBKT)-yKNN] (0.1 <= x <= 0.6 and 0 <= y <= 0.05) ceramics were prepared with compositions both at as well as off the morphotropic phase boundary (MPB) of x = 0.2 and y = 0. It was found that KNN addition destabilizes a ferroelectric order in BNT-BKT at zero electric-field, resulting in low remanent polarization, remanent strain and effective small signal d(33,eff) [23 pC/N for (x, y) = (0.2; 0.05), 60 pC/N for (0.2; 0.02)], while it does not affect the maximum polarisation and strain at high fields and thus enhances large signal piezoelectric coefficient, d*(33) [200 pm/V for (0.2; 0.05), 217 pm/V for (0.2; 0.02)]. This effect of KNN was found to be independent from the MPB and thus equally high d*(33) values were obtained for compositions at and far off the MPB. Thus, adding KNN to BNT-BKT removes the restriction to remain at exactly MPB position and provides more flexibility while offering equal or even improved strain. (C) 2011 The Japan Society of Applied Physics