This paper describes an apparatus for the measurement of thermal conductivity and reports the thermal conductivity of poly(organosiloxane) liquids. Experimental results are presented for the thermal conductivity of nineteen poly(organosiloxane) liquids (molecular mass from 272 to 14 887 g/mol) of three homologous series (five poly(ethylsiloxane)s, ten poly(methylsiloxane)s, and four poly(phenylmethylsiloxane)s) at atmospheric pressure in the temperature range of 293 K to 623 K. The maximum relative measurement error is +/- 1.5%.