The temperature dependence of magnetic domain pattern in titanomagnetite and titanohematite of self-reversing andesites from the 1985 eruption of the Nevado del Ruiz volcano (Colombia) has been observed using the Bitter technique. During heating/cooling cycles in the laboratory, the natural remanent magnetization loses the self-reversal properties between 110 ¿C and 180 ¿C. The titanohematite domains disappear in the same temperature interval. Large grain regions without visible colloid contrast are observed between room temperature and 180 ¿C. Utilizing the high resolution of a magnetic force microscope, it is found that these regions contain either domains with wall spacing of about 3 &mgr;m or a buckling pattern. Along the phase boundary between a titanomagnetite and a titanohematite grain but within the titanohematite, a 2 &mgr;m wide chemically different domain has been observed. It has a higher iron content than the main titanohematite. The two titanohematite phases represent two phases with higher Curie temperature, Tc, but lower spontaneous magnetization, Ms, and lower Tc but higher Ms, respectively. Negative interaction between these phases is called upon to explain the self-reversing mechanism. ¿ American Geophysical Union 1993