Small angle neutron scattering (SANS) was used to investigate the structural changes of a 5 wt % asphaltene solution in perdeuterated l-methylnaphthalene (1MN-d(10)) as a function of temperature. A special stainless steel cell was constructed and used for the measurements. The SANS data measured at various temperatures from 20 to 400 degrees C show that the scattering intensity continuously decreases with increasing temperature. The data at 20 degrees C suggested rodlike morphology for the particles, and a nonlinear curve in the Guinier plot implied polydispersity in their sizes. Maximum entropy analysis using the form factor for a cylinder allowed extraction of particle size distributions in the radius and length space. At 20 degrees C the asphaltenes self-associate in 1MN-d(10) forming long rod-shaped particles whose radius was around 18 Angstrom but vary in length over 500 Angstrom. At 50 degrees C these aggregates break down, as evidenced by the decrease in signal intensity and the radius of gyration. In the temperature range of 100-320 degrees C the maximum length of the particles decreases and the polydispersity varies in both the radius and length dimensions. Between 340 and 400 degrees C, the particles become smaller having a spherical shape with a radius around 12 Angstrom. Upon returning the sample to 20 degrees C, the SANS signal was too weak to derive any structural information, implying irreversible thermochemistry. We also carried out similar studies on the deasphalted oil (DAO) in 1MN-d(10) at 20, 100, and 200 degrees C, but the SANS signals were very weak and no changes in the scattering behavior were seen as a function of temperature.