Thermal conductivities of water-agar gels were determined at concentrations with mass fractions between w = 0.001 and w = 0.1 and temperatures from (278.15 to 323.15) K by means of a specifically designed probe system based on the line heat source principle. The thermal conductivity probe was calibrated and validated using glycerin (with mass fraction w = 0.99), glycol (with mass fraction w = 0.99), and ultra pure water (with resistivity 18.4 M Omega.cm, electric conductivity 0.055 mu S.cm(-1)). Their measured and recommended values of thermal conductivity were in good agreement. The experimental data of thermal conductivity values of water-agar gels were sensitive to both concentration and temperature. The thermal conductivity of water-agar gels decreased with increasing concentration, while it increased as temperature increased. Predictive regression models were proposed to give a good fit for thermal conductivity with concentration and temperature.