Single-crystal X-ray analysis of 1-(2,4,6-tri-tert-butylphenyl)-3-methylphosphole synthesized to test the effect of the strongly sterically demanding P-substituent on the geometry of the molecule, revealed that the phosphorus pyramid was drastically flattened; the normal out of plane angle of 65 degrees (formed between the P-substituent and the ring C-2-P-C-5 plane) was reduced to 45.9 degrees. Consistent with strong electron delocalization, the C-3-C-4 bond length was dramatically shortened relative to that for other phospholes, and the Bird index of aromaticity was 56.5, almost the equivalent of that in pyrrole (59). The phosphole ring, normally resisitant to electrophilic substitutions, underwent reaction with acetyl chloride-aluminum chloride, consistent with considerable cyclic electron delocalization.