It assumed the name Hesse und bei Rhein in to distinguish itself from the Electorate of Hesse , which had formed from the neighbouring Landgraviate of Hesse-Kassel. Colloquially, the grand duchy continued to be known by its former name of Hesse-Darmstadt.
The country was promoted to the status of grand duchy and received considerable new territories, principally the Duchy of Westphalia. After the French defeat in , the grand duchy joined the new German Confederation. Westphalia was taken by Prussia , but Hesse received Rhenish Hesse in return. A constitution was proclaimed in and a long process of legal reforms was begun, with the aim of unifying the disparate territories under the grand duke's control.
The political history of the grand duchy during this period was characterised by conflict between the conservative mediatised houses Standesherren and forces supporting political and social liberalisation. During the revolutions , the government was forced to grant wide-ranging reforms, including the full abolition of serfdom and universal manhood suffrage, but the reactionary government of Reinhard von Dalwigick rolled most of these back over the following decade.
In , Hesse entered the Austro-Prussian War on the Austrian side, but received a relatively mild settlement from the Prussian victors.
The grand duchy joined the German Empire in As a small state within the empire, the grand duchy had limits placed on its autonomy, but significant religious, social, and cultural reforms were carried out. The portion of the grand duchy on the right bank of the Rhine stretched most of the way from the south of the modern state of Hesse to Frankenberg. The portion on the left bank was located in the modern state of Rhineland-Palatinate. In addition to the great floodplains of the Rhine Hessian Ried , Main , and Wetterau , the grand duchy also contained upland regions like the Vogelsberg , the Hessian Hinterland , and the Odenwald.
