ChemGlobe: Holmium
- Electronic, thermal, and steric data along with an isotope table.
- http://partisans.spurious.biz/~pkremer/projects/chemglobe/ptoe/_/67.html

LANL: Holmium
- Basic information, history, and properties.
- http://periodic.lanl.gov/elements/67.html

Wikipedia: Holmium
- Properties of the element, including its history, applications, and characteristics.
- http://en.wikipedia.org/wiki/Holmium

Lenntech: Holmium
- Physical data, chemical properties, health and environmental effects.
- http://www.lenntech.com/Periodic-chart-elements/Ho-en.htm

EnvironmentalChemistry.com: Holmium
- Atomic structure, chemical and physical properties, and table of nuclides.
- http://environmentalchemistry.com/yogi/periodic/Ho.html

It's Elemental: Holmium
- Basic physical and historical information.
- http://education.jlab.org/itselemental/ele067.html

ChemicalElements.com: Holmium
- Basic information, atomic structure, and isotopes table.
- http://www.chemicalelements.com/elements/ho.html

Visual Elements: Holmium
- Image, source, uses, key isotopes, and ionisation energies.
- http://www.chemsoc.org/viselements/pages/holmium.html