Ammonia released from laying hen excreta compromises bird and worker health and adds reactive nitrogen to the atmosphere, which has stimulated interest in inexpensive adsorbents made from crop residues. This study evaluated corncob-derived activated carbon (CCAC) applied as a surface amendment to fresh laying hen excreta. Corncobs were sun- and oven-dried, carbonised in a low-oxygen drum kiln, ground to pass a 60-mesh sieve and chemically activated with 1 M H3PO4; the product contained 11.2% moisture and 6.6% ash. Fresh excreta (50 g) received CCAC at 0 (P0), 10 (P1), 15 (P2) or 20% (P3) of excreta weight in a completely randomised design with five replicates and were incubated in closed jars for 7 days at ambient temperature. Residual ammonia (Conway microdiffusion), pH (1:10 suspension) and mass loss were determined. CCAC did not affect residual ammonia (1.95–2.98 mg NH3/g fresh sample; P = 0.342). Excreta pH was higher with CCAC than without it (8.81–8.88 vs. 8.59; P = 0.030; control vs. CCAC contrast P = 0.005). Mass loss declined linearly with CCAC level (P < 0.001), from 1.49 g (2.98% of initial substrate mass) in P0 to 0.66 and 0.60 g (1.14 and 1.00%) in P2 and P3, which did not differ from each other. Accordingly, 15% CCAC lowered 7-day mass loss by about 56% and raised excreta pH, but it did not reduce the total ammoniacal pool retained in the excreta. Corncob activated carbon thus acted mainly as a moisture- and volatile-retaining amendment rather than as an ammonia-removing agent.