Voltage-dependent scanning tunneling microscopy is used to determine the buckling of pi-bonded chains on Si and Ge(111)2 x 1 surfaces. Images are acquired over a wide range of voltages, and are compared with theoretical constant-density contours generated from first-principles electronic-structure calculations. The theoretical predictions for <2 (1) over bar(1) over bar > corrugation shifts are quite different for positive and negative buckling; experimental results for Si are found to agree with the former and those for Ge agree with the latter. In addition to an expected shift in <2 (1) over bar(1) over bar > corrugation between small-magnitude positive and negative voltages, a further shift is also seen in both experiment and theory between small and large positive voltages. (C) 2004 American Vacuum Society.