The influence of bias voltage on the quality of scanning tunneling microscopy images of carbon nanotube “rafts” was investigated in the range from −1 to 1 V in combination with scanning tunneling spectroscopy (STS) measurements. While for positive tip polarity only a slight voltage dependence was found in the image quality, for negative polarity a strong increase of the noise was observed with increasing voltage. STS showed that, for negative tip polarity, the tunneling current may be different in different locations by several orders of magnitude. ©1998 American Institute of Physics.