| IBM | Niwot | CO |
| Ideal Cement | Denver | CO |
| Ideal Cement | Portland | CO |
| Ideal Cement | Boettcher | CO |
| Immaculate Conception Cathedral | Denver | CO |
| Industrial Service & Engineering | Denver | CO |
| Industrial Sugar | La Salle | CO |
| International Rubber | Denver | CO |
| Iron Silver Mining | Leadville | CO |
| James Richards | Del Norte | CO |
| Jardine Douglas | Colorado Springs | CO |
| J.C. Street John Plumbing & Heating | Colorado Springs | CO |
| J D Insulations | Denver | CO |
| Jefferson County Courthouse | Golden | CO |
| Jefferson School | Pueblo | CO |
| Jewish Consumptives Relief Society | Denver | CO |
| Job A Cooper | Denver | CO |
| John Dunn | Boulder | CO |
| John Evans Junior High School | Greeley | CO |
| Johns Manville | Denver | CO |
| Johns Manville | Florence | CO |
| Johns Manville | Littleton | CO |
| Johns Manville | Ken Caryl | CO |
| Juanita Coal & Coke | Bowie | CO |
| Kendrie & Bolthoff Manufacturing & Supply | Denver | CO |
| Kindle & Angelo Sheetmetal | Denver | CO |
| Kitayama Brothers | Brighton | CO |
| Kodak | Windsor | CO |
| Kohl Elementary School | Broomfield | CO |
| Kuner Pickle | Brighton | CO |
| La Bella Mill, Water & Power | Goldfield | CO |
| La Combe Power Plant | Denver | CO |
| Lacombe Station | Denver | CO |
| Lamar Light & Power Department | Lamar | CO |
| Lamar Power Plant | Lamar | CO |
| Landmark Petroleum | Fruita | CO |
| Larimer County Hospital | Fort Collins | CO |
| Larimer Light & Power | Fort Collins | CO |
| Lauren Burt | Denver | CO |
| Leadville Gas & Electric | Leadville | CO |
| Leadville Light & Power | Leadville | CO |
| Leyden Gas Plant | Leyden | CO |
| Lindrooth Shubart & Company | Denver | CO |
| Liquid Carbonic | Denver | CO |
| Littleton Creamery | Denver | CO |
| Longero | Denver | CO |
| Longmont Beet Sugar | Longmont | CO |
| Longmont Junior High School | Longmont | CO |
| Loretto Heights College | Denver | CO |
| Louisiana Pacific | Fruita | CO |
