The Gysbert van Steenwyk, Sr. Materials contain correspondence, financial records, C.C. Washburn estate, organizations, and legal documents. The correspondence includes business as well as personal correspondence particularly from van Steenwyk's nephew Ernest F. C. Klokke. The business correspondence dates from 1863-1902 and includes letters to and from van Steenwyk. Among the people with whom van Steenwyk corresponded are U. S. Congressman Jeremiah M. Rusk and Wisconsin governors Edward Salomon and Cadwallader C. Washburn. Much of the correspondence from the three men mentioned deals with the construction of a railroad bridge at La Crosse.

The financial records of Gysbert van Steenwyk, Sr. date from 1850-1903, the year of the final distribution of his estate. With the exception of a list of Batavian Bank depositors and La Crosse Lumber Co. tax book, the records document his personal finances, including land purchases and stocks, bonds, and rental property records.

A third group of records relate to van Steenwyk's role as one of the executors of the Cadwallader Colden Washburn estate. C. C. Washburn was a Civil War general, governor of Wisconsin, lumber baron, and industrialist. The materials consist of correspondence, including some letters from Washburn's daughter, Jeanette (Washburn) Kelsey; financial records, including a general inventory of the estate and a summary of its assets; and legal documents produced in the course of settling the estate, especially relating to provisions made for Washburn's invalid wife, Jeanette.

Financial records of organizations in which van Steenwyk, Sr. was active make up another series. They consist of a treasurer's book from the Brotherhood of St. Andrew, and a cash book and ledger from the La Crosse Theater Co. A collection of legal documents, including van Steenwyk's commissions as Dutch Consul for Wisconsin, Minnesota, and Michigan, and warranty deeds make up a fifth series.

Finally, there are a number of miscellaneous items such as an 1841 journal written in Dutch with the translated title, "Writings About Diverse Subjects in Alphabetical Order," a speech given by van Steenwyk at the end of his term as mayor (1873), and minutes of transit railroad meetings. Sadly an undated travel diary is missing from the collection having never been returned from an exhibit loan.