Effects of thermal annealing at 400 °C on the optical, structural, and chemical properties of {\mathrm{TiO}}_2 single-layer, {\mathrm{MgF}}_2 single-layer, and {\mathrm{TiO}}_2/{\mathrm{MgF}}_2 narrow-bandpass filters deposited by conventional electron-beam evaporation (CE) and plasma ion-assisted deposition (PIAD) were investigated. In the case of {\mathrm{TiO}}_2 films, the results show that the annealing of both CE and PIAD {\mathrm{TiO}}_2 films increases the refractive index slightly and the extinction coefficient and surface roughness greatly. Annealing decreases the thickness of CE {\mathrm{TiO}}_2 films drastically, whereas it does not vary that of PIAD {\mathrm{TiO}}_2 films. For PIAD {\mathrm{MgF}}_2 films, annealing increases the refractive index and decreases the extinction coefficient drastically. An x-ray photoelectron spectroscopy analysis suggests that an increase in the refractive index and a decrease in the extinction coefficient for PIAD {\mathrm{MgF}}_2 films after annealing may be related to the enhanced concentration of \mathrm{MgO} in the annealed PIAD {\mathrm{MgF}}_2 films and the changes in the chemical bonding states of \mathrm{Mg}2p , \mathrm{F}1s , and \mathrm{O}1s . It is found that ({\mathrm{TiO}}_2/{\mathrm{MgF}}_2) multilayer filters, consisting of PIAD {\mathrm{TiO}}_2 and CE {\mathrm{MgF}}_2 films, are as deposited without microcracks and are also thermally stable after annealing.