The Lewis acid promoted reaction of 1,1-diarylallenes with ketone derivatives was examined. The tin(IV) chloride promoted reaction of diarylallenes with vinyl ketones gave indene derivatives through a conjugate addition/cyclization reaction. The reaction of diphenylallene with diethyl oxomalonate in the presence of one equivalent of tin(IV) chloride at –40 °C gave diethyl hydroxy(3-phenyl-1<i>H</i>-inden-2-yl)malonate as the major product through a carbonyl addition/cyclization reaction, whereas the same reactants in the presence of 0.2 equivalents of tin(IV) chloride at 80 °C gave diethyl (3-phenyl-1<i>H</i>-inden-2-yl)malonate. Diethyl hydroxy(3-phenyl-1<i>H</i>-inden-2-yl)malonate was also converted into the latter product on heating at 80 °C in the presence of 0.2 equivalents of tin(IV) chloride.