To find the sum of the series \(1.5 + 2.5 + 3.5 + 4.5 + \ldots + 8.5\), we can recognize that this is an arithmetic series where each term increases by a constant difference.

First, identify the first term (\(a\)) and the common difference (\(d\)):
- The first term \(a = 1.5\)
- The common difference \(d = 1\) (since \(2.5 - 1.5 = 1\))

Next, identify the last term (\(l\)) of the series:
- The last term \(l = 8.5\)

Now, we need to determine the number of terms (\(n\)) in the series. The \(n\)-th term of an arithmetic series can be found using the formula:
\[ a_n = a + (n-1)d \]

Set \(a_n = 8.5\) and solve for \(n\):
\[ 8.5 = 1.5 + (n-1) \cdot 1 \]
\[ 8.5 = 1.5 + n - 1 \]
\[ 8.5 = n + 0.5 \]
\[ n = 8.5 - 0.5 \]
\[ n = 8 \]

So, there are 8 terms in the series.

The sum \(S_n\) of the first \(n\) terms of an arithmetic series can be calculated using the formula:
\[ S_n = \frac{n}{2} \cdot (a + l) \]

Substitute the values we have:
\[ S_8 = \frac{8}{2} \cdot (1.5 + 8.5) \]
\[ S_8 = 4 \cdot 10 \]
\[ S_8 = 40 \]

Therefore, the sum of the series \(1.5 + 2.5 + 3.5 + 4.5 + \ldots + 8.5\) is \(40\).