The effect of parallel barriers on the collector efficiency of flat-plate solar air heaters has been investigated theoretically and experimentally. The barriers were placed with uniform spacing and in parallel, thereby dividing the air channel (collector) into parallel subchannels (subcollectors) of the same size. These subcollectors were connected in series so that air flowed through them in sequentially reversed directions. Experimental studies were performed for different locations of the barriers. The theoretical predictions agree reasonably well with experimental results. The optimal barrier location for maximum collector efficiency is the center line of the collector. The collector efficiency increases theoretically as the number of barriers increases.