Observations of electric field-gradient induced birefringence (the Buckingham effect) in gaseous C6H6 and C6F6 over ranges of temperature (approximate to 318-415 K) and pressure (up to approximate to 63 kPa) are reported. Analyses of the data show that at 300 K the temperature-independent contributions to the effects are not negligible (C6H6, -9.1 +/- 1.3%; C6F6, 7 +/- 5%) and definitive molecular quadrupole moments are obtained [C6H6, Theta = (- 30.4 +/- 1.2) x 10(-40) C m(;)(2) C6F6, Theta = (28.3 +/- 2.1) x 10(-40) C m(2)]. Electric and magnetic properties of C6H6 and C6F6 derived from electro-optical and magneto-optical experiments are collated and compared in relation to the charge distributions in these species.