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The need for flexible interconnects in advanced applications in consumer electronic products is increasing rapidly. The reliability and flexibility of ultra-thin chip-on-flex (UTCOF) interconnects formed using anisotropic conductive adhesive (ACA) are thus investigated. Two films of ACA materials, namely ACA-P and ACA-F, are assembled at different bonding temperatures to study the effect of temperature on the adhesion at the substrate-adhesive and adhesive-chip interfaces using differential scanning calorimetry (DSC) and a 90° peeling test. The contact resistance of a daisy chain with 188 input/output (I/O) is measured to examine the quality of bonding through dummy test samples with an 80-μm pitch. The reliability of the fabricated UTCOF interconnects bonded via selected ACA joints is evaluated by performing an 85°C/85% RH thermal humidity storage test (THST) for 1000 h, and their flexibility is evaluated in static bending and four-point bending tests. The interfaces between the ultra-thin silicon chip and the substrate of failed samples in the THST and four-point bending testing are then investigated by scanning electron microscopy (SEM), which is utilized to obtain cross-sectional images. Finite element analysis is also conducted to elucidate the failure mechanism of the UTCOF interconnects in the four-point bending test. The averaged maximum allowable deflections of the fabricated UTCOF interconnects with ACA-P and ACA-F materials are 26% and 168%, respectively, higher than those of the COF interconnects with a chip thickness of 670 μm. Moreover, the contact resistance remains stable, varying by less than 10%, in the static bending test with a bending radius of 30 mm. According to the results thus obtained, give the appropriate choice of an ACA material and the optimal curing conditions, the UTCOF interconnects with ACA joints reliably serve as flexible interconnects for use in consumer electronic products.