An experimental study of the critical heat flux (CHF) in horizontal and vertical tubes cooled with R-134a has been completed. The investigated ranges of flow parameters in R-134a were outlet pressures of 1.31, 1.67 and 2.03 MPa (8, 10 and 12 MPa in water-equivalent values), mass flux from 500 to 3000 kg m(-2) s(-1) (700-4300 kg m(-2) s(-1) in water-equivalent values), and critical quality from -0.1 to +0.9. The wide range of qualities was achieved using tubes of different heated lengths and two-phase flow at the test-section inlet. The R-134a CHF data obtained in the vertical orientation agreed with the R-134a-equivalent CHF values from the water-based CHF look-up table. The effect of orientation on CHF was found to depend on mass flux, quality and pressure, as well as the limiting critical quality. This effect is strong at low mass fluxes, but disappears at high mass fluxes. At qualities higher than the limiting critical qualities, the CHF in horizontal flow can be greater than the corresponding value in vertical flow at the same critical quality conditions. A maximum reduction in CHF due to flow stratification was observed at qualities between the limiting critical qualities for horizontal and vertical flows. The orientation effect on CHF appears to be much stronger for R-134a than for water flow at the same critical quality, equivalent mass flux (based on vertical flow fluid-to-fluid modeling relationships) and density ratio. This behavior is primarily due to the larger density difference between liquid and vapor and the lower vapor velocity in R-134a.