Evaluation of the sustainability of biomass pyrolysis requires a thorough assessment of the product yields and energy densities. With this purpose, a laboratory scale fixed bed reactor (FBR) was adapted from the standard Gray-King (GK) assay test on coal to conduct fixed bed pyrolysis experiments on agricultural and agro-industrial by-products. The present study provides results on the pyrolysis of two types of biomass: chipped olive tree trimmings (OT) and olive pomace (OP). Solid (char) and liquid (tar) product yields are reported. Mass yields are determined and compared with values obtained in similar works. Results indicate that char yield decreases from 49% (OT-db) and 50% (OP-db) at 325 °C to 26% (OT db) and 30% (OP-db) at 650 °C. Tar yield is almost constant (42%) at different reaction temperatures for OT, while it decreases slightly from 42% to 35% for OP. Energy density of the products at different peak temperatures is almost constant for OT (1.2), but slightly increases for OP (from a value of 1.3 to a value of 1.4).