Class A asking rental rates in the Boston office market may be nearing bottom, largely because of a 5 percent quarter-over-quarter decrease in total available space in the third quarter, according to a report from commercial brokerage giant Grubb & Ellis.

Class A asking rental rates declined 54 cents quarter-over-quarter to $35.95 per square foot in the June-September period, the company found. A similar report issued recently by competitor FHO Partners pegged asking rents at $39.75 in Boston.

"However, the fact that total available space decreased by 5 percent from the previous quarter may indicate that rental rates will soon reach their low point," Grubb & Ellis said in a statement.

Overall, office vacancy decreased to 14.5 percent in the third quarter, down 20 basis points from the second quarter’s vacancy rate of 14.7 percent. The region posted positive net absorption of 660,000 square feet, representing the largest amount of space absorbed by the market in two years, the company said.  Total net absorption year-to-date is approximately 310,000 square feet.

"Recent economic news indicates recovery has begun, although the 14.5 percent vacancy rate remains high by any historical measure," the company said in a third quarter summary. "The good news is Massachusetts has experienced seven straight months of job growth, and the state continues to outpace the national recovery, which bodes well for the Boston-area office market."