We study the finite-temperature effective potential for a λΦ4 theory with a tachyonic mass. More precisely we compute the contribution from multiloop graphs with nonoverlapping bubbles. This is done by substituting the one-loop effective mass with an auxiliary field χ which has to be self-consistently determined. The two phases of the system are characterized by different values of the order parameters ϕ and χ. The temperature behaviour of the effective mass and coupling constant are discussed in some detail.