Halo growth was in situ observed in the unidirectionally solidified transparent organic camphor-naphthalene eutectic system, in an attempt to experimentally examine the origin and characteristic of halo which has been frequently found in certain off-eutectic alloys. The tip temperatures of the constituents of the alloy - camphor halo, naphthalene plate and eutectic - were measured with varying growth velocity. The coupled region boundary on the naphthalene side of the eutectic composition was determined from the relation between the growth velocity and growth temperature. On the basis of the experimental results the relation was examined between the halo growth and the coupled region boundary which had been previously suggested. The halo growth temperature is much higher than expected. It seems plausible that the nominal compositions of the interdendritic liquid correspond to those on the coupled region boundary, but in effect halo growth is not likely to occur simply for that compositon. Halo growth temperature was found to be independent of the alloy composition. This is an expected result from the proposed model.