  We study the effects of CP violation on the nature of the chiral transition
within the linear sigma model with two flavors of quarks. The
finite-temperature effective potential containing contributions from nontrivial
values for the parameter $\theta$ is computed to one loop order and their
minima structure is analyzed. Motivated by the possibility of observing the
formation of CP-odd domains in high-energy heavy ion collisions, we also
investigate the behavior of the effective potential in the presence of a strong
magnetic background. We find that the nature of the chiral transition is
influenced by both $\theta$ and the magnetic field.
