An investigation is made into the finite-temperature behavior of a class of scalar field theories derived from string field theory. The fields {tilde {phi}} are the lowest-order fields in a level truncation of the open bosonic string and have momentum-dependent couplings. Expressions for the one-loop effective potential for {phi}{sup 3} and {tilde {phi}} {sup 3} theories are derived, and the finite-temperature mass corrections are examined for indications of a second-order phase transition. Questions concerning the stability of the vacuum, the number of degrees of freedom, and the Hagedorn temperature are addressed.