Flow boiling in a straight, rectangular channel was investigated to explore the conditions leading to critical heat flux (CHF). The flow channel had a 5.0 x 2.5 mm cross-section and 101.6 mm heated length, heat was applied to the shorter dimension along only one wall. Tests were performed with FC-72 liquid over a velocity range of 0.25 to 10 m s(-1) and at outlet subcoolings of 3, 16 and 29 degrees C. Video images captured at CHF at various flow conditions revealed that vapor coalesces into a series of patches resembling a wavy vapor layer which propagates along the heated wall allowing liquid to contact the wall only at discrete locations. Measurements indicate that vapor patch length, vapor patch height and liquid contact length increase along the flow direction but decrease with increasing subcooling and velocity. The measurements also reveal that the ratio of liquid-to-vapor length remains approximately constant along the heated wall for a particular subcooling. These observations and measurements represent key contributions to critical heat flux modeling efforts presented in Part II of this study.