The diffusion decay of a pulsed low-pressure discharge plasma in an electronegative gas is theoretically studied assuming the absence of plasmachemical processes. According to the previously established mechanisms, evolution of the plasma parameters involves two stages in which the variation of both density and energy of electrons is controlled by the transport processes. In the first stage, the flux of negative ions to the wall is absent and the electron density decays sharply with time, whereby almost all electrons escape within a finite time from the discharge volume where an ion - ion (electron-free) plasma is formed. In the second stage, this plasma decays by an ion - ion ambipolar diffusion mechanism.