Iron oxide nanotubes of 50-150 nm outer diameter and 2-20 nm wall thickness are prepared in ordered arrays. Atomic layer deposition (ALD) of Fe
2
O
3
from the precursor iron(III) tert-butoxide at 130-180 °C yields very smooth coverage of the pore walls of anodic alumina templates, with thickness growth of 0.26(+/- 0.04) A per cycle. The reduced Fe
3
O
4
tubes are hard ferromagnets, and variations of the wall thickness d(w) have marked consequences on the magnetic response of the tube arrays. For 50 nm outer diameter, tubes of d(w) = 13 nm yield the largest coercive field (H-c 750 Oe), whereas lower coercivities are observed on both the thinner and thicker sides of this optimum.