An aryl p-(trifluoromethyl) substituent increases the affinity of 1,3-disubstituted 8-phenylxanthines at A2a-adenosine receptors, while having little effect on affinity at A1-adenosine receptors. In contrast, an aryl p-(trifluoromethyl) substituent has little effect on affinity of 3,7-disubstituted and 1,3,7-trisubstituted 8-phenylxanthines. An aryl p-sulfo substituent reduces affinity of all 8-phenylxanthines at A1- and A2a-adenosine receptors. An 8-(trifluoromethyl) substituent markedly reduces affinity of 1,3-dialkylxanthines at both A1- and A2a-adenosine receptors. In contrast, 8-(trifluoromethyl)caffeine retains affinity for A2a-adenosine receptors, but does lose affinity for A1-adenosine receptors. 8-Bromo-, 8-acryl-, and 8-pent-1-enylcaffeines are also selective for A2-adenosine receptors, while 8-cyclobutylcaffeine is nonselective. 8-[trans-2-(tert-butyloxycarbonyl)vinylcaffeine is 20-fold selective for Aza vs A1 receptors.