This work evaluates the volumetric mass transfer coefficient (k(L)a), the gas hold-up (epsilon) and the mixing time (t(m)) as a function of superficial gas velocity (U-G) in a flat-panel photobioreactor (PBR) with high light path. CO2 utilization efficiency and volumetric power consumption (P/V) were also evaluated. A 50 L working volume photobioreactor was developed, 0.67 m in length, 0.57 m in height and 0.15 m in width (light path). The height-width ratio was 3.8, which is lower than reported in most PBRs. Initially, experiments were performed with air and tap water (biphasic system) and, subsequently, using a Spirulina sp. culture (triphasic system: air. Culture medium, cells). Minimum and maximum superficial gas velocity values were 5 x 10(-5) and 8.4 x 10(-3) m s(-1), respectively. Maximum values for k(L)a and epsilon were 20.34 h(-1) (0.0057 s(-1)) and 0.033 in the biphasic system, and 31.27 h(-1) (0.0087 s(-1)) and 0.065 in the triphasic system. CO2 utilization efficiency was 30.57%. Results indicate that the hydrodynamic and mass transfer characteristics of this photobioreactor are more efficient than those reported elsewhere for tubular and other flat-plate PBRs, which opens the possibility of using PBRs with higher light paths than yet proposed. (C) 2009 Elsevier B.V. All rights reserved.