Abstract A microbit apparatus has been used to study how and why field drilling rates are usually higher when air is circulated rather than liquids. Our laboratory results with air are compared with the work of others who have circulated liquids to remove cuttings, and a theory for air drilling has been developed. Chief results of this program are that most of the parameters operative in liquid drilling are also operative in air drilling, but in air drilling many of these factors are much less detrimental to the drilling rate. Our major conclusions are that drilling rates with air are much greater than with liquids because chip removal is facilitated by:low bottom-hole pressure exerted by the column of air,low viscosity of air which permits it to enter cracks more readily,less restriction to widening crack by highly compressible air, andlow cohesive forces between rock particles in environment of air. Furthermore, there is far less plugging of widening cracks around chips because of the absence of solids from the drilling fluid and the better hole cleaning afforded by the high-velocity air stream. The greatest reason for high drilling rates with air is that the air is greatly cooled by expansion as it passes through the bit and thereby cools the bottom of the hole to reduce the stresses exerted on the rock by the overburden. Overburden pressure, liquid saturation of the rock and rock ductility have a more adverse effect on gas drilling than drilling with liquids. The effect of overburden pressure is greatest on dry rocks.