Impedance spectroscopy enables fast and nondestructive dielectric characterization of petroleum fluids during different stages of exploration and processing. The majority of existing studies focused on frequencies above 10(3) Hz. The advantage of considering smaller frequencies is that more information on the electrical conductivity of the sample can be extracted. In this work, dielectric relaxation measurements on Boscan asphaltenes and maltenes are performed by impedance spectroscopy in the low-frequency range of 40-10(6) Hz. The data are analyzed within the formalism of complex impedance. As frequency increases, the system shifts from highly conducting below similar to 10 Hz to highly insulating above 105 Hz. A comparison of impedance with electric modulus and permittivity suggests that only one primary relaxation mechanism exists between 10 and 101 Hz because of conductance effects. The alternating-current (AC) conductivity is almost independent of frequency below 105 Hz and increases with increasing frequency above this range. Extrapolation of AC conductivity to zero frequency provides an estimate of direct-current (DC) conductivity, from which the molecular size of asphaltenes can be determined. In addition, the onset of asphaltene aggregation can be detected from the variations of DC conductivity with asphaltene concentration in toluene. Thus, low-frequency dielectric relaxation is a simple and powerful tool to predict asphaltene-associated problems in the field.