The influence of soda lignin and thiolignin on the mechanical properties (tensile strength and elongation at rupture) as well as on the dielectric properties (conductivity (sigma), permittivity (epsilon') dielectric loss (epsilon") and dielectric relaxation time (tau)) of their composites with natural rubber (NR) was investigated. All measurements were carried out on NR, NR/soda lignin (20 phr), and NR/thiolignin (20, 30, and 40 phr) composites. The mechanical properties reveal that NR/thiolignin composite possesses the best thermal stability and 20 phr is the optimum thiolignin loading. The dielectric study was carried out over a frequency range from 100 Hz to 100 kHz at temperature range from 20 to 80 degrees C. Dielectric data were fitted in the frequency domain using Havriliak-Negami and Frohlich functions in addition to the conductivity term. The different relaxation mechanisms in the system were also discussed according to these functions. (c) 2005 Wiley Periodicals, Inc.