In this article the force transmissibility of a quasi-zero-stiffness (QZS) isolator is considered. The isolator comprises a vertical spring and two oblique springs that are either linear, linear with pre-stress or softening nonlinear with pre-stress. The force transmissibility of such a system is derived and compared with that of a linear system. Assuming light damping, simple approximate expressions for the maximum transmissibility and jump-down frequencies are derived. It is shown that there are advantages in having nonlinear and pre-stressed oblique springs compared to the other configurations, and that all the QZS systems can outperform the linear system provided that the system parameters are chosen appropriately.