The performance of first generation steam assisted gravity drainage (SAGD) plants for bitumen recovery has improved through operational experience, but there remain ample opportunities for the introduction of technologies that can further improve energy efficiency and plant reliability. Laboratory testing and validation is an important initial step in technology development. A major factor that determines the applicability and validity of testing results is the integrity of process water samples used in lab and field studies for this purpose. The results presented in this paper demonstrate aging of SAGD process water and its direct implication on membrane performance testing. Aging in samples collected after primary bitumen/water separation occurred mainly through reactions of dissolved organic species with air. This resulted in a gradual change in appearance, accompanied by a significantly higher tendency to foul membranes in dead-end filtration tests. The root cause for this change was proposed to be the reaction of phenolic species with oxygen, leading to more compressible and tightly packed filter cakes on the membrane surface. This effect was mitigated by minimizing air exposure during sample collection and handling. These results establish that preventing oxygen exposure to the sample is critical for maintaining sample integrity during a test program. Although this study focuses on filtration, aging effects can also lead to misleading results in laboratory testing of other water treatment processes and must be carefully considered during technology evaluation and development.